Sir Keir Starmer’s government is facing claims of a cover-up after it emerged that messages between his chief adviser and the former peer Peter Mandelson are missing.

Morgan McSweeney’s phone, which contained the messages, was stolen last year.

On Sunday, cabinet minister Steve Reed said that the theft had been reported to police months before the scandal involving the one-time Labour grandee erupted.

But the Tories said the situation “stinks of a cover-up”.

Sir Keir has been forced to pledge that a huge swathe of information about Mandelson’s appointment as US ambassador will be published in the wake of revelations about his relationship with the paedophile financier Jeffrey Epstein.

Mr McSweeney, who is reported to have pushed for Lord Mandelson to get the ambassadorial role, resigned as Sir Keir’s chief of staff earlier this year, after a row around his appointment and what was known about the pair’s links at the time erupted.

Tory frontbencher Alex Burghart said: “We had to drag the Mandelson files out of Keir Starmer and now we find the phone of his former chief of staff and protege of Mandelson won’t be part of the disclosure. The whole thing stinks of a cover-up.”

Housing secretary Steve Reed told LBC on Sunday: “His phone was stolen and he reported it at the time.”

It is understood that the Cabinet Office does hold some of the messages that passed between Mr McSweeney and Lord Mandelson.

Sir Keir has repeatedly said that he is sorry for believing Mandelson’s “lies” and for appointing him to the key role in Washington. He has also made a frank apology to the victims of Epstein for the decision.

Lord Mandelson was arrested on February 23 on suspicion of misconduct in public office, having been accused of passing sensitive information to Epstein during his time as business secretary.

He was subsequently bailed, but later released from his bail conditions, although he remains under investigation.

Last month, Wes Streeting became the first senior Labour figure to publish his private messages with Lord Mandelson, revealing his criticism of Sir Keir’s government.

They also revealed that the health secretary believed he “is toast at the next election” and that, as one of Sir Keir’s most senior ministers, he lambasted the government for having no economic growth strategy and policy on Israel’s “war crimes”.

A government spokesman said: “We are committed to complying with the Humble Address in full, while continuing to support the Metropolitan Police with their investigation.”