Eurosceptics pronounced the controversial Lisbon Treaty dead today after it was thrown out by voters in Ireland.
Gordon Brown faced immediate demands to halt the process of ratifying the treaty as the latest attempt to reform the European Union was thrown into turmoil.
Critics insisted a 'No' vote in the Irish referendum represented the same fatal blow to the Lisbon Treaty as was dealt to the EU Constitution by French and Dutch voters in 2005.
The Prime Minister of Luxembourg Jean-Claude Juncker has already said that ratification of the agreement will not go ahead in January 2009 as planned.
Vote for the future: Counting begins in Dublin this morning
The Irish No campaign won by 862,415 votes to 752,451, giving them 53.4% of the vote.
The announcement in Dublin sparked wild celebrations among 'No' campaigners.
But in Brussels, European Commission head Jose Manuel Barroso called for other states to continue their ratification processes and said a solution should be sought.
Mr Barroso said he had spoken to Irish Prime Minister Brian Cowen and agreed with him that this was not a vote against the EU.
'Ireland remains committed to a strong Europe - ratifications should continue to take their course,'' he added.
France and Germany quickly issued a joint statement expressing regret over the Irish result.
European leaders earlier said they had no "plan B" for how to proceed if Ireland's electorate voted No.
More than half of Ireland's 43 constituencies returned a No vote in the referendum.
Downing Street declined to comment on reports that the Prime Minister had called French president Nicolas Sarkozy to assure him that British ratification would continue.
The House of Lords this week cleared the way for Parliament to ratify the treaty when it rejected a Conservative bid to force a referendum.
But prominent eurosceptic David Heathcoat-Amory insisted today the Government must 'go back to the drawing board' and respect the voters of Holland, France and Ireland.
'Of course (ratification) must be halted,' the Tory MP told BBC Radio 4's The World At One.
Celeration: Supporters of the 'No' applaud as they watch the results of the Irish referendum
It's a no: Justice Minister Dermot Ahern
'The Bill before the British Parliament must be withdrawn because under the European Union's own laws this cannot proceed now. The Lisbon Treaty is dead.
'When the French and Dutch voted "no" in 2005 the British Government did halt that particular Bill.
'If they don't do the same this time it'll be quite clearly because they want to put pressure on Ireland and the people of Ireland to change their mind.'
Open Europe, a think tank which opposes the treaty, warned Mr Brown it would be 'electoral suicide' to proceed with ratification without regard for the views of the public.
Ministers have steadfastly refused to grant a British referendum on the treaty, despite promising to grant one on the now-defunct Constitution.
They say that the two documents are substantially different, and that a plebiscite on the treaty is unnecessary, but eurosceptics insist they are the same in all but name.
Ireland is the only country holding a referendum on the treaty, which supporters maintain is essential to streamline decision-making after the EU's expansion to 27 member states. All 27 must ratify the document.
Open Europe's director, Neil O'Brien, said: 'This is a resounding victory on behalf of ordinary people across Europe over an out-of-touch and arrogant political elite.
'If supporters of the EU Constitution cannot even win in Ireland - one of the most pro-EU countries in Europe - it is clear that their vision for the future of Europe is now discredited in a most fundamental way.
'It would be tantamount to electoral suicide for Gordon Brown to ignore the Irish result and try and force through the Lisbon Treaty regardless - as the British Government has already said it would.
Stark mesage: A woman walks past graffiti in Dublin. Irish voters rejected the European Union's new reform treaty in a crunch referendum
'The argument for a referendum in Britain is now overwhelming.'
Conservative MEP Daniel Hannan hailed the apparent rejection of the treaty as a 'victory for Ireland and for Irish democracy'.
'People have defied the hectoring and bullying of almost all their political parties, almost all the Irish media, and political leaders from across Europe,' he said.
'They have struck a blow for freedom throughout Europe. Truly, today, we are all Irish.'
Bob Crow, general secretary of transport union the RMT, stepped up demands for a British referendum in response to the Irish result.
'The EU Constitution, whatever it may now be called, is an anti-democratic privatisers' charter and would have undermined the right of working people to defend their jobs,' he said.
'The Irish referendum result is a massive victory for democracy and the Constitution is now dead in the water, despite our own Government's attempt to railroad it through without the referendum the British people were promised.'
Spreadbetting tycoon Stuart Wheeler, who is awaiting a High Court verdict on his legal challenge demanding a referendum, said: 'The winner of today's vote in Ireland is democracy.
'Regardless of the result, the people of Ireland have at least been given a voice.
'Gordon Brown, on the other hand, has denied the people of Britain the chance to have their say on Lisbon.
'This is a vote that the overwhelming majority of people in the UK have repeatedly demanded.'
Many economists attribute Ireland's long boom to its relationship with the European Union. The country's economy grew at about six per cent per annum throughout the 1990s, far above other European Union member countries.
More than 70 per cent of the country's exports go to EU member states, while EU subsidies and grants make up two per cent of Ireland's GDP.