LONDON - Thousands of foreign prisoners may be allowed to stay in Britain on their release from jail, despite Gordon Brown's pledge to crack down on overseas offenders, the Conservative Party said on Friday.

The Tories said a leaked Prison Service memo reveals that immigration officials have "no interest" in deporting those who have spent less than a year behind bars.

Shadow Justice Secretary Nick Herbert said it means 4,000 foreign criminals convicted each year will avoid deportation.

"The result will be that foreign thieves, fraudsters, burglars and drugs dealers will be released back into the community," Herbert said.

The Border and Immigration Agency's Chief Executive Lin Homer said its policy was to target the most dangerous prisoners first by deporting those jailed for more than year.

"But that does not mean others are slipping through the net," Homer said. "We are increasingly deporting those with shorter sentences.

"The UK Borders Act will allow us to ensure that deportation is automatically considered for anyone who commits a serious offence no matter how long their sentence."

Brown's spokesman could not immediately be reached for comment.

Prisoner numbers have reached record levels of around 80,000 in recent years, leading to some criminals being released early to ease overcrowding.

Two prisons in England are being used solely for foreign nationals to try to speed up the process of deportation, Justice Minister David Hanson said in October.

Brown said in a newspaper interview in July that he wanted to send a clear message to migrants: "If you commit a crime you will be deported from our country. You play by the rules or you face the consequences."

The Tories' Herbert accused Brown of "a special kind of cynicism" for pledging to take tough action against overseas prisoners, while allowing many to stay in Britain.