About 600,000 people have come to work in the UK from eight nations which joined the European Union in 2004, says Home Office minister Tony McNulty.
Bulgaria and Romania are set to join the EU in 2007
New figures show that 447,000 people from Poland and the seven other new EU states have applied to work in the UK.
But Mr McNulty said the figure would be nearer 600,000 if self-employed workers - such as builders - were included.
He said the migrants were helping the UK economy, but the figures will fuel fears about strains on public services.
427,000 workers from eight EU accession states successfully applied for work in UK
Over half (62%) are Polish
82% are aged 18-34
56% work in factories*
Anglia region has highest proportion of workers (15%)
Source: Home Office
All figures May 2004-Jun 2006 except *Jul 2004-Jun 2006
And they will increase debate about whether the right to work should be limited when Bulgaria and Romania join the EU.
The figures show those 427,000 migrant workers who successfully registered to work brought with them 36,000 dependents - spouses and children. Some 27,000 child benefit applications were approved.
The government predicted there would be 15,000 people a year from the new EU member countries moving to the UK for work.
But between May 2004 and June 2006 the government approved just over 427,095 work applications.
More than half of those - 264,560 - are from Poland. The other migrants on the worker registration scheme are from the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Slovakia and Slovenia.
Mark Boleat, from the Association of Labour Providers, suggested another 100,000, workers or possibly a lot more, had arrived without registering.
The government says the migrant workers are helping to fill gaps in the UK's labour market, especially in administration, business and management, hospitality and catering.
It is believed low salaries in Poland, sometimes around £200 a month, is one reason the country's citizens are attracted to working the UK.
Ursula Gacek, an opposition member of the Senate in Warsaw, who is drafting legislation to encourage Poles to return home, said some recruiters were already offering Poles in London more lucrative salaries.
"I was speaking to a colleague today, and he's recruiting for staff in London, amongst Poles, to come back to Wroclaw, at salaries which are closer to the £500 mark than the £200 or so," she said.
At the moment, there is little sign of a slowdown in the migration flow, with more than 50,000 registering to work in Britain between March and June this year.
The Home Office says they are "making few demands on our welfare system".
As well as those claiming child benefit, 680 people were told they could get income-related benefits.
Next EU members
Mr McNulty denied the government had grossly underestimated the number of migrants - it had only predicted the number of people who would settle in the UK.
"These are economically productive individuals who are making significant contributions to our economy," he told BBC News 24.
When the last tranche of countries joined the EU in 2004, the UK was one of only three countries to give full work rights to their citizens.
But Trade and Industry Secretary Alistair Darling appeared to signal at the weekend that the government might agree to new curbs, promising there would be no "open door" policy.
But Mr McNulty said the government had not yet decided whether to impose restrictions on migrant workers from Romania and Bulgaria, which are due to enter the EU in 2007.
The Conservatives say ministers should learn from the "unprecedented numbers" who arrived in the UK after the last EU expansion and impose restrictions on workers from the two countries.
Shadow immigration minister Damian Green said: "These figures make it all the more urgent that the government takes an early decision and stops ducking and diving on the issue of Bulgaria and Romania entering the EU."
Liberal Democrat spokesman Ed Davey said there had been difficulties but migrants had brought many positive benefits.
"Jobs that weren't being done are now being done, productivity improvements, the skills gap in certain sectors being met," he said.
"So I hope we can have a rational debate, a more mature debate and not one that's driven by hysteria."
Former Labour minister Frank Field the number of migrants was unmanageable and made it increasingly difficult for local people to get jobs.
Sir Andrew Green, chairman of Migration Watch UK, said the case for restrictions for Romanians and Bulgarians was now "unanswerable".
The latest figures also show asylum applications fell 24% in 2005 to at 25,710, excluding 5,130 dependants. A total of 69,000 applications, including dependents, were approved.
Between in March and June this year the number of failed asylum seekers removed reached its highest ever at 5,070, including dependants.
There were 5,490 applications, excluding dependants over the same period.