The loss of the personal data of thousands of prisoners by a Home Office contractor could prejudice the trials of some of the country’s most dangerous criminals, lawyers and opposition politicians have warned.

Dominic Grieve, the shadow home secretary, said that if information about previous convictions was placed on the internet, suspects could argue that their chance of a fair trial had been jeopardised and so escape justice.

His concerns were echoed by lawyers at the campaign group Liberty, which warned that cases could collapse.

“The last thing we want is people who are guilty being freed because the government has jeopardised their trials by the loss of data,” said Gareth Crossman, a criminal lawyer and director of policy at Liberty.

“This is yet another incident of outrageous data loss, which shows that this government cannot be trusted to handle sensitive information.”

Last week PA Consulting told the Home Office that it had mislaid a memory stick containing the names, dates of birth and expected release dates of 84,000 inmates, as well as further information, including the previous convictions of 33,000 repeat offenders.

The Tories said they would demand a statement from Jacqui Smith, the home secretary, as well as answers from Jack Straw, the justice secretary, who is in charge of prisons.

The Home Office is reviewing all contracts held by PA Consulting. The firm has close links with government departments and has hired General Sir Mike Jackson, the former chief of the general staff, as a senior adviser.

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