Downing Street has again been accused of a cover-up in the Peter Mandelson scandal, after a senior advisor who vetted the former US Ambassador had their phone stolen – which means text messages involving former ambassador Lord Mandelson may never be recovered. The phone belonged to Morgan McSweeney, Sir Keir’s former chief of staff and a close associate of the disgraced peer, who quit Number 10 last month after critics placed the blame for the Mandelson appointment squarely on his shoulders.

Furious MPs expressed disbelief that the critical information may now be out of reach, with Alex Burghart, the Shadow Chancellor of the Duchy of Lancaster, saying that “important material must be properly transferred onto official systems” according to official guidance. Whitehall Mandarins expect that “sensitive Government business should not be conducted on non-corporate devices”, he added.

Speaking to the Daily Express, Mr Burghart said: “Last week the Prime Minister refused to answer the most basic questions about what he did. This week Number 10 is refusing to provide even the most basic verifiable details about his Chief of Staff’s phone.” The top Tory lamented that the “whole business stinks from nose to tail” as he repeated calls for the “straight answers, not excuses”.

The Government has been forced to release thousands of documents relating to the controversial appointment of the former US Ambassador, after it emerged he had maintained a close friendship with convicted American paedophile Jeffrey Epstein.

Earlier this month it emerged in the first tranche of the Mandelson-files that swathes of information were “missing” from the dossiers. It included a section on the peers official vetting file, which marked him as a reputation risk and made reference to his friendship with Epstein, which had been left empty.

The box, reserved for comments from the Prime Minister – which former No10 advisors told the Express needed to have filled in – appears empty. Downing Street denied accusations made by Conservative Leader Kemi Badenoch of a cover up.

More documents are expected to be released, but no official timeline has been given as to when they are due.

A government spokesman said that “all government departments, ministers and relevant individuals have been asked to provide the information they hold within scope of the humble address”.

They did not address concerns about the potentially missing messages from Mr McSweeney’s phone, which was stolen before the demand to hand over the messages was made, directly.

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

Lord Mandelson denies any wrongdoing.