Shadow chancellor Ed Balls has asked police to investigate how personal documents left in his former Whitehall offices were leaked to the Daily Telegraph.
Balls reported the theft of the material to Scotland Yard last week and force is now 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.
Balls, a former schools secretary, believes the files were taken from his old department – now renamed the Department for Education.
A spokesman for 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.”
He confirmed that 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.”
Meanwhile, Chancellor George Osborne today refused to say whether he had prior knowledge that the newspaper planned to run stories based on private papers belonging to Mr Balls.
Kevin Brennan, who was a minister in Balls’ department, challenged the Chancellor during a debate on the economy, asking: “Did you have advanced knowledge that the Daily Telegraph had obtained the shadow chancellor’s private papers, or any advance knowledge of the stories they planned to write?”
Osborne said: “This is a debate about the economy, and we all enjoyed reading those papers in the Daily Telegraph.”
Balls also questioned the Chancellor, saying the issue was “a serious matter”.
He asked Osborne: “Did you have any advance knowledge, or sight of, papers taken from me which went to the Daily Telegraph without my knowledge? I would like you to answer the question.”
Osborne replied: “We’ve all read those papers in the Daily Telegraph.”