The Government was forced to admit last night that it has underestimated the number of migrants eligible to claim benefits and work in Britain for the past seven years.
Officials privately concede that at least 50,000 migrants have slipped though the net each year since 1997 sparking renewed criticisms of the Government's competence.
Ministers are also being accused of attempting to hide a large increase in the numbers by delaying the publication of official statistics.
The delay - which is being blamed on "concerns over the quality of the statistics" - coincided with the resignation of Beverley Hughes as Immigration minister. Ms Hughes quit amid allegations that visa controls had been improperly loosened to prevent a surge of visa applications fromEU accession states.
In the weeks following her departure, Andrew Smith, Secretary of State for Work and Pensions, stopped the publication of migrant worker figures showing what one civil servant admitted was a "very large increase".
Last night officials insisted that the delay was due to the fact that this year's figures included many workers who should have been included in previous years' statistics.
The number of migrantsentitled to take up jobs and claim benefits in Britain has been rising fast. In 1997, the official number was 130,300 but by 2002 the total had risen to 156,400. The figures for this year due out at the end of April will not now be published until the autumn.
David Davis, the shadow Home Secretary, last night accused ministers of attempting to "smuggle through" a large increase in the number of migrants eligible to claim benefits.