Some Polish workers in Britain are illegally claiming child benefit in both the UK and their home country, a BBC investigation has found.
Officials in a number of Polish regions told Five Live Report up to half the applications they see are fraudulent.
They added checks on claims were under strain because of the numbers involved.
EU rules allow migrant workers to claim benefit for children who are in a different country, but they cannot do so in two countries at once.
Two years ago in the Gdansk region of Poland, officials were asked to check just two applications for British child benefit. So far this year they have processed more than 1,500 claims.
The BBC collated information from nine of Poland's 16 regions.
Six of those regions also provided estimates of the number of double claimants - in each case saying these represented at least one third and up to one half of total claims.
Benefits system
The UK government estimates that it loses approximately ?2bn to benefit fraud ever year - and the Polish child benefit claims would be just a small part of that total.
Eastern European migrant workers have limited access on arrival to the welfare state, other than to Child Benefit which is a universal payment.
Of the 600,000 workers who had registered in the UK over the last two years, some 16,868 had applied for some form of income support but the vast majority were turned down.
Seven out of 10 Eastern European workers who have registered in the UK since 2004 are Polish.
Proportionally fewer Poles have applied for income-related benefits than other Eastern European arrivals.
But former welfare minister Frank Field said it was time for a serious reassessment of migrant workers' access to benefits.
"That's not what the point of having a welfare state in this country was originally about. Nor do I think it is what it should develop into," he said.
'System buckling'
Agnieszka Zablocka, who checks the validity of applications for child benefit made in the UK by workers from the Gdansk region in northern Poland, says that many of the double-claimers think that the British authorities will never discover their real situation.
"It usually takes place in small towns. People think, 'How will the UK ever know. How would they ever find out?'," she says.
She says there are so many fraudulent double-claim cases because the system which processes the applications is buckling under the sheer weight of numbers.
Ms Zablocka's department in Gdansk had just one employee in 2004 but now has nine, and she says they are still struggling to cope.
Because of the flood of applications by Poles for child benefit in the UK, she says that it can take up to two years to check their validity.
She also blames Britain's "pay now, check later" policy for contributing both to the scale of the fraud and the time it has taken for it to be detected.
HM Revenue and Customs, which is in charge of recovering the money, said in a statement: "HMRC takes all allegations of fraud extremely seriously and is looking carefully at the controls in place to ensure all claims are legitimate.
"The UK is working very closely with all its European partners, including Poland, to meet our obligations under EU rules and prevent fraud. We have robust procedures in place."