The blunder emerged after a computer was stolen from the Communities Secretary's constituency office in Salford at the weekend.

Despite strict procedures introduced across Whitehall in the wake of data scandals last year, the PC - thought to contain restricted material on extremism, defence and the housing market - is not believed to have been encrypted.

The top civil servant at Communities and Local Government, Peter Housden, said in a statement: "It is clear that papers have been sent to Hazel Blears in a way that is not fully consistent with the departmental guidance."

However, he stressed the computer was password protected, insisting that "no damage had been done" because the documents were not "classified as secret or top secret".

Department sources indicated that no officials were likely to lose their job over the blunder, but did not rule out disciplinary action.

Ms Blears is also likely to face questions over why she was not aware that the PC's security was inadequate for viewing such material.

The blunder is the latest in a string of recent lapses in the handling of government information.

Last week a Cabinet Office official was suspended and an investigation launched after secret papers featuring details of al Qaida and security in Iraq were left on a train.

Home Secretary Jacqui Smith is to be hauled before the Commons Home Affairs Select Committee after plans to tackle funding for global terrorism were also found on a train.