Shadow chancellor Ed Balls has called in police to investigate how personal documents left in his former Whitehall offices were leaked to the press.
Mr Balls reported the theft of the material to Scotland Yard last week. The force said it was "assessing" whether to launch an investigation.
The Daily Telegraph ran a series of stories earlier this month based on the documents, including details of negotiations over when Gordon Brown would succeed Tony Blair as prime minister.
The former schools secretary believes the files were taken from his old department - now renamed the Department for Education.
A spokesman for Mr Balls said: "The theft of Ed Balls's private documents, which the Daily Telegraph published this month, has been reported to the Cabinet Secretary, the House of Commons authorities and the police.
"The Cabinet Secretary and the police are both now in the process of investigating this matter."
The spokesman confirmed that Mr Balls had asked the newspaper to return the material.
A Metropolitan Police spokesman said: "We can confirm that we have received an allegation of theft from an individual.
"This allegation is currently being assessed."