The PC belonging to Hazel Blears, Communities Secretary, was stolen from her constituency office in Salford, Manchester. It is thought to contain restricted information on extremism, defence and the housing market, and it is thought the machine's data is not encrypted.
Peter Housden, permanent secretary for Communities and Local Government, 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."
He claimed no damage had been done because the documents were not classified as secret or top secret. He said the machine was password-protected.
Blears may now face questioning over why the PC's security was inadequate for viewing such material.
Her office had a security upgrade five years ago when she became a Home Office minister with responsibility for counter-terrorism.
The theft is yet another embarrassment for the Government, following two leaks of secret data last week, both of which were left in the form of documents on trains running into and out of London's Waterloo station.