The Home Office has drawn up a secret list of more than 45,000 "undesirables" from Bulgaria and Romania who may attempt to enter the UK when the two countries join the European Union next year.
The Government privately estimates that between 60,000 and 140,000 Romanians and Bulgarians will arrive in Britain within a year of accession, according to a leaked document. The paper, by Home Office Minister Joan Ryan, warns that they may include people who pose a criminal or security risk.
Once the two eastern European states are members of the EU, it will be much more difficult for individuals who pose a threat to be turned away, as they will no longer have to obtain a visa but will simply show their passports on arrival.
The warning list of potential undesirables has been compiled by the Immigration and Nationality Directorate from records of Bulgarians and Romanians who have previously tried to enter the UK, states the paper.
But Ms Ryan said that more information should be sought from the governments of the two countries involved - known as the A2 or "Accession 2" states.
The Government has already faced criticism for under-estimating the influx of migrants from the ten countries, including eight from eastern Europe, which joined the EU in 2004.
Ministers predicted that just 13,000 eastern Europeans would arrive each year, but a recent report from London University researchers put the total at 600,000 over two years - half of them from Poland.
Ms Ryan warned of the danger of "enlargement fatigue" if a new wave of A2 nationals arrives next year, suggesting that it would add to the public perception that Britain had accepted enough immigrants.
She said Ministers must decide by October whether to impose restrictions on work permits for A2 nationals. Concerns over organised crime are far greater for Romania and Bulgaria than for the previous new entrants.
Earlier this year, the European Commission said that Bulgaria must do more to crack down on corruption, organised crime, money laundering and people trafficking if it was to go ahead with its planned entry on January 1 2007. Romania was less harshly criticised.