Migrant worker numbers shot up to 1.5 million last year after the UK opened its labour market to eastern Europeans from the new EU states, academics said today.
This influx is "almost certainly the largest ever single wave of immigration the British Isles have ever experienced", according to immigration expert Professor John Salt.
Many of these new immigrants are Poles, who represent "the largest ever single national group of entrants" to the UK, he said.
The research is published today in a booklet for the Economic and Social Research Council by Prof Salt, of University College London, and Professor Phil Rees, of Leeds University.
Numbers of foreign workers in the UK topped a million for the first time in 1998, and by last year this figure had reached 1.5 million, or 4.1% of the total workforce.
Prof Salt argues this large rise is connected to Britain's decision to give citizens of the 10 new EU member states - Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia - access to its labour market in May 2004.
In 2004 an estimated 223,000 more people migrated to Britain than moved abroad - significantly up from an estimated net influx of 151,000 the previous year.
Prof Salt said: "Opening up of the labour market to citizens of the new member states of the EU from May 2004 initiated what is almost certainly the largest ever single wave of immigration the British Isles have ever experienced, with Poles the largest ever single national group of entrants."
In the past, foreign workers in the UK have generally been more skilled than the domestic workforce, but this is changing, probably because of immigration from eastern Europe, he added.
Prof Rees predicted that numbers of white people in Britain would grow only slightly between now and 2020 because of low fertility rates and smaller numbers of women of child-bearing age.
He noted signs that ethnic minority groups were moving from economically weaker northern cities to the more prosperous south of England.
By 2020 outer London will take over from inner London as the most important region in the country for ethnic minorities, he forecast.