The number of East Europeans coming to work in Britain since 2004 may be 50% higher than previously thought, a BBC Two Newsnight survey suggests.
The UK population topped 60m last year
The survey of 500 Poles in the UK found 64% had signed the workers' register.
Official figures show 375,000 workers have registered since the EU expanded in May 2004 but the survey suggests 187,000 more may have come to the UK.
The Tories said it showed ministers had "dramatically" underestimated the scale of immigration from new EU countries.
'Pressure on housing'
Shadow home secretary David Davis said: "This survey implies that the number of Eastern European immigrants is over half a million, much more than what the government now claim and dramatically more than the 13,000 a year that they originally forecasted.
"This will put an incredible level of pressure on housing and public services and is a much higher level than the government calculated for when it entered into this short-sighted open borders policy."
The government originally estimated just 15,000 people a year would migrate to the UK from Eastern Europe when it agreed to admit workers from the new accession countries such as Poland, Czech Republic, Estonia, Hungary and Latvia.
But among the Polish workers questioned, 30% said they had not signed the workers' register and the remaining 6% had never heard of it.
The survey, by the Centre for Research on Nationalism, Ethnicity and Multiculturalism at the Universities of Surrey and Roehampton, also indicated many more Polish people were intending to stay in Britain long-term or even permanently than previously thought.
More than 40% said they wanted to stay at least two years, and 15% - more than one in seven - had already decided to move here permanently.
Only one in three definitely intended to go home within two years. Nearly a third were planning to bring their families to Britain or had already done so.
According to latest government figures, 374,555 from the eight nations - including 228,235 Poles - registered for work here between May 2004 and the end of March this year.
The survey found that unemployment and low wage rates were the main factor driving Poles to migrate to the UK, and more than 40% said it was easier to live in Britain than Poland.
A third also said Britain offered better opportunities for personal and professional development.
Of those surveyed about 90% said they had been received well or very well in the UK.
A further two Eastern European countries are due to join the EU on 1 January 2007.
The UK government has yet to indicate whether workers from Romania and Bulgaria will enjoy the full working rights associated with being an EU citizen from the outset.
Last week former Labour minister Frank Field sparked controversy when he questioned whether current levels of immigration into the UK were sustainable.
In an interview with the BBC News website, he called for politicians on all sides to stop ignoring the issue before it was exploited by the far right.
But on Tuesday Tony Blair rejected calls by Labour MP Tony Wright, chairman of the Commons public administration committee, for an independent inquiry into the costs and benefits of mass immigration.
The prime minister said any debate should be about how best to control immigration, not about overall numbers.