Part of the explanation for that may be the extent to which Government ministers can seem determined to sow confusion about what those facts are. Last week, for instance, Phil Woolas, the Immigration Minister, insisted at a hearing of the Commons Committee on Home Affairs that the ONS predictions were wrong: the number of migrants would diminish in the near future.
It is not clear what basis he had for making that claim. He certainly did not provide any evidence for it – which may explain why there is almost no expert in the field who agrees with him. It is now more or less universally recognised that the net inflow of people to Britain (the number who arrive, minus the number who leave) every year is 190,000, and there is no reason to think that figure will diminish in the near future unless the Government takes drastic action.
It means that the population of Britain will be 70 million by 2028. Frank Field and Nicholas Soames, two MPs from the Cross-Party Group on Balanced Migration, are unequivocal about the seriousness of the situation, even if Mr Woolas is not. "We are," they say, "facing a population crisis."
WHY IS IMMIGRATION INTO BRITAIN SO HIGH?
The reason why people from developing countries want to live here is obvious: life is better here than in the impoverished countries from which they came. But that is true of all the developed economies of Europe. All are much better places in which to live than the nations of Africa, for instance, or the Indian subcontinent.
So what explains why so many immigrants target Britain, and why there are such queues at the Sangatte centre in France to get across the Channel to England?
Word has got out that Britain is more hospitable to immigrants than other European countries – which it is. The legacy of empire means we are more used to receiving them and are more willing to integrate them. One of the great virtues of the British people is their tolerance. Our society is much less closed to outsiders than most countries of continental Europe. It is easier to get a job here – less restrictive employment legislation means it is easier to fire people, so employers are more willing to hire migrants they can pay less in the first place. British employers are less suspicious of foreigners than their counterparts in countries such as Italy or France.
Add the fact that it is easier for immigrants who arrive here to claim benefits, get council housing and access health and education services for themselves and their children and it becomes clear why Britain is a target for migrants.
WHAT IS THE IMPACT OF ASYLUM?
Understandably, many migrants are desperate and will use any means they can to ensure they can settle here. From 1997 to 2002, it seemed that claiming asylum – claiming that you are a refugee fleeing persecution – was the most effective way of gaining the right to settle here. In 2002, 84,000 people claimed asylum. Add their dependents and the number was more than 100,000. The majority had their claims rejected, but very few of them were deported: most were allowed to stay, even if they were not given an officially recognised right to do so.
Since then, the Government has taken steps to tighten up the asylum system, and last year fewer than 22,000 people claimed asylum in Britain. But the proportion of asylum-seekers who are deported after their claim is rejected remains tiny, so asylum-seekers remain a significant source of migrants.
WHAT ARE THE ELECTORAL ADVANTAGES OF IMMIGRATION FOR LABOUR?
The migrants' desire to get into Britain is only half the story of why so many are here. The other half is the explanation of why the Government decided to let so many migrants into Britain. For the past 12 years, Labour has been convinced that allowing high levels of immigration should be a policy priority, and it has taken steps to ensure that it is relatively easy for migrants to get permanent, legally recognised residence here. Why has Labour done that?
In 1997, Tony Blair and his Cabinet had several reasons for wishing to increase the number of immigrants into Britain. One was that they perceived an electoral advantage. The outcome of elections in an increasing number of constituencies was, and is, dictated in large part by the votes from relatively recent immigrants. People who have moved here from, say, the Indian subcontinent understandably hated the rules that prevented their families joining them in Britain.
The most onerous of those rules was known as "the primary purpose rule", which was imposed by the Conservatives in 1993. It required that someone wishing to follow his or her spouse into Britain to prove that "the marriage was not entered into primarily to obtain admission to the UK". Proving a negative, as the rule obliged the candidate to do, was extremely difficult, and large numbers of spouses were refused entry into Britain as a result.
Labour abolished that rule soon after the election in 1997. The move was extremely popular in immigrant communities, because it made it far easier for families to move here. Immigration by spouses into Britain has increased by 50 per cent since the primary purpose rule was abolished. More than 40,000 people were granted citizenship here on the basis of marriage in 2008 alone.
WHAT ARE THE ECONOMIC EFFECTS OF IMMIGRATION?
Equally important – indeed, perhaps more important – in persuading Labour to adopt a much more relaxed immigration policy was the belief among many ministers and advisers that Britain's economy would flourish if only there were to be large-scale immigration here. When he was Home Secretary, David Blunkett said there was "no natural limit" to the number of migrants that Britain could absorb. And his successor, Charles Clarke, was also frank about his desire for "more immigrants".
Government reports and press releases trumpeted the miraculous economic benefits of immigration: high levels of immigration would boost prosperity very significantly by increasing Britain's gross domestic product (the measure of how much the country produces every year); it would ensure an end to labour and skills shortages; it would enable the NHS and other public services to grow at a faster rate by using cheaper immigrant labour; and it would help Britain to avoid the "pensions time-bomb" created by an ageing population by adding a whole new tier of youthful and energetic workers.
That is why Labour drastically increased the number of work permits it issued to migrants wishing to come to Britain. In 1997, 40,000 work permits were issued. More than 130,000, or over three times as many, were issued in 2008.
The consensus that migration offers large economic benefits to Britain has proved to be wrong on every point. A committee of the House of Lords, made up of economists and ex-finance ministers, published a definitive investigation into the issue almost exactly one year ago, on April 1 2008. The report, The Economic Impact of Migration, stated bluntly that "overall GDP, which the Government has persistently emphasised, is an irrelevant and misleading criterion for assessing the economic impacts of immigration on the UK. The total size of an economy is not an index of prosperity. The focus of analysis should be on the effects of immigration on income per head of the resident population. Both theory and the available empirical evidence indicate that these are small."
The additional income generated by immigrants is not much bigger – and may in fact be smaller – than the number of people they have added to the UK's population, so they do not increase prosperity to any significant degree.
The Lords' report went through the other claims made for the economic benefits for immigration and dismantled them one by one. "Immigration is unlikely to be an effective tool for reducing labour and skill shortages," the report stated, providing copious evidence for its claim. It stressed that immigrants, although they performed an important role within public services such as the NHS, were not in fact necessary to the functioning of those services: native British labour could have, and still could, perform the same role.
The report pointed out that immigration cannot defuse "the pensions time-bomb", since immigrants themselves grow old and need pensions. And it concluded that there was "no evidence for the argument made by the Government that immigration generates significant economic benefits for the existing UK population".
In fact, there is some evidence that immigration means that the least-educated portion of the workforce is less well-off, since immigration provides a competing supply of labour prepared to do the least pleasant and worst-paid jobs for even less money.
WHAT ABOUT THE OTHER EFFECTS OF IMMIGRATION?
Labour ministers seem to have given very little thought to other, non-economic consequences of large-scale immigration: the pressure on schools, the NHS and other public services.
The increased pressure on housing, particularly council housing, has been an enormous source of resentment for native-born Britons who thought they were in line for a council house, only to be bypassed in favour of recent immigrants, because the officials allocating council houses decided that those immigrants had greater needs: they had more children.
There hasn't been an in-depth study of the effect of immigration on council housing, but the effect on the availability of council houses to native, working-class Britons has certainly been perceived by that group to be dire.
WHAT ARE THE SOCIAL EFFECTS OF IMMIGRATION?
These are very hard to assess because they are hard to measure. But one result familiar from research into communities in the US that have experienced a sudden influx of large numbers of immigrants has been a diminution of trust. Immigrant communities tend not to mix, either with other immigrants or with residents of the host country. The result is a mosaic of tightly knit groups that do not integrate with each other and do not trust each other. The idea that the nation is a community based on reciprocal obligation suffers.
The texture of communities changes. Some people like those changes. Others do not. The changes are not confined to the appearance of ethnic restaurants or men and women who wear different clothes. An influx of people who are fundamentalist about their religion, for example, can mean that some of them do not accept ideas that mainstream Britain takes for granted: the primacy of secular over divine law; the equality of women with men; and the importance of the freedom to change your religion without persecution. How to ensure that immigrants adopt "British values" is a problem the Government has thought long and hard about – but has not solved.
WHAT SHOULD BE DONE NEXT?
The Government appears to be in the process of recognising that a continued influx of migrants at the rate of 190,000 every year for the indefinite future is not going to be beneficial to Britain. Ministers have introduced a points system for the granting of work permits. The trouble with that system is that it will only reduce the number of migrants by 5 per cent a year – and if the Government wants to stop Britain's population from reaching 70 million in the next 20 years, it will have to reduce the number of migrants by 75 per cent.
What could be done? Parliament's Cross-Party Group on Balanced Migration suggests that economic migration should be clearly separated from settlement. At the moment, after five years of working in Britain, you have the right to settle here permanently. The Cross-Party Group recommends abolishing that right, which would have a significant impact on numbers.
Three other policies that would have an effect would be: re‑introducing some version of the primary purpose rule to diminish the number of spouses coming to Britain; increasing the number of failed asylum-seekers that are deported; and increasing the checks and controls on our borders.
None of these policies will be easy to implement, and all of them will have some very harsh consequences for would-be migrants to Britain, their spouses and their children. But the party that adopts them will gain electoral support: the evidence is that the British people are extremely concerned about the present levels of immigration and want to see them drastically reduced.