Downing Street staff will be able to give anonymous evidence suggesting Boris Johnson lied to Parliament over partygate, MPs ruled as they began their key investigation.

The cross-party Privileges Committee on Wednesday unanimously backed Labour grandee Harriet Harman to chair the investigation that could determine Mr Johnson’s fate as Prime Minister.

The committee said it expects oral evidence sessions to begin in the autumn, meaning the inquiry will hang over Mr Johnson’s head for months.

They issued a call for witnesses to come forward to help determine whether he deliberately misled MPs when he denied rule-breaking parties had been taking place in No 10.

Harriet Harman
Harriet Harman (Niall Carson/PA)

Evidence suggesting Mr Johnson had knowledge of the gatherings during the coronavirus pandemic or of any inquiries he made about them were suggested as relevant to the inquiry.

Subject to Ms Harman being able to identify the individuals in discussions with committee staff, the panel determined it would be willing to take written or oral evidence from people who wish to remain anonymous.

Ms Harman has twice led Labour in opposition as acting leader and is the Mother of the House as the longest-standing female MP.

Sir Ernest Ryder, a former lord justice of appeal, was selected to advise the inquiry.

The MPs gave until July 29 for witnesses to come forward and evidence to be presented, saying it can be submitted to the committee website.

Mr Johnson survived a vote of no confidence by Tory MPs earlier this month in the wake of Sue Gray’s damning inquiry into partygate.

Result of the vote of confidence in Boris Johnson as Conservative leader
(PA Graphics)

But 148 Conservatives – or 41% of his party – voted against Mr Johnson, in what was a major blow to his authority.

Under the current rules of the Conservative 1922 Committee, the Prime Minister cannot face another vote over his leadership for a year.

However, the group of backbench MPs could work to change this if they believe any events change the situation and that he should go.

Some Conservatives have said they believe Mr Johnson being found to have lied to the House, normally considered a resigning matter, would cross that line.

Ms Harman was widely expected to be chosen as the chair, after Labour’s Chris Bryant stepped aside to ensure fairness because of his previous criticism of Mr Johnson.

The 71-year-old MP for Camberwell and Peckham, in London, was first elected in 1982.

Her husband, the Labour MP Jack Dromey, died suddenly in January.

The committee is comprised of seven MPs, four Tories, two from Labour and one from the SNPs.

Their investigation will consider Mr Johnson’s statements including, but not limited to, him insisting in December last year that “all guidance was followed in No 10”.

“I have been repeatedly assured since these allegations emerged that there was no party and that no Covid rules were broken,” is another.

As is Mr Johnson insisting: “I am sickened myself and furious about that, but I repeat what I have said to him: I have been repeatedly assured that the rules were not broken.”