Pledging to introduce tougher controls for asylum seekers, she said: "There is no case, in the national interest, for immigration of the scale we have experienced over the last decade."
In a strongly worded statement, the Institute of Directors (IoD) accused Mrs May of putting internal party politics ahead of Britain's best interests.
IoD director general Simon Walker said: "We are astonished by the irresponsible rhetoric and pandering to anti-immigration sentiment from the Home Secretary.
"It is yet another example of the Home Secretary turning away the world's best and brightest, putting internal party politics ahead of the country, and helping our competitor economies instead of our own.
"The myth of the job-stealing immigrant is nonsense. Immigrants do not steal jobs, they help fill vital skill shortages and, in doing so, create demand and more jobs.
"If they did steal jobs, we wouldn't have the record levels of employment we currently do.
"It is about time the Home Office stopped undermining business and our own government's efforts to secure productivity growth.
"Political leaders should stop vilifying migrants and acknowledge the hugely important contribution they make to this country's economy."
Mrs May's speech was warmly received in the conference hall, but political opponents, commentators and charities questioned the factual basis and tone of her address.
A political editor at the traditionally Tory-supporting Daily Telegraph newspaper said the speech was "awful, ugly, misleading, cynical and irresponsible".
Labour's Diane Abbott accused Mrs May of getting "down in the gutter with UKIP chasing votes for her leadership bid".
Refugee Council Chief Executive Maurice Wren said: "The Home Secretary's clear intention to close Britain's border to refugees fleeing for their lives is thoroughly chilling, as is her bitter attack on the fundamental principle enshrined in international law that people fleeing persecution should be able to claim asylum in Britain."
NWN: And here is a major problem. Big business just loves cheap labour ! That means bigger profits for themselves. They care not for a nation or it's people. As long as they rake in the money to alleviate their greed. This is why 'big business' has to be controlled.