Former Conservative minister Nick Gibb said he has handed in a letter, writing in The Daily Telegraph: "We need to face up to hard truths. To restore trust, we need to change the Prime Minister."

Aaron Bell, MP for Newcastle-under-Lyme, earlier said the partygate allegations and the prime minister's subsequent handling of the fallout has made his position "untenable".

Politics live: Resignation of Downing Street official 'will be terminal' for PM - as Tory MP says public has been 'let down'

How many Tory MPs have called on the PM to resign?

A total of 13 Tory MPs have now publicly called for the PM to go, with eight revealing they have submitted letters of no confidence in him.

Mr Johnson will face a vote of confidence in his leadership if 54 Conservatives - 15% of the party's 360 MPs - send letters to Sir Graham Brady, chairman of the 1922 Committee, demanding a contest.

Mr Bell, who was first elected in 2019, is the fifth MP this week to announce they have lost confidence in the PM and submitted a letter to Sir Graham.

He spoke in the Commons on Monday as Mr Johnson faced MPs in the wake of the release of a partial version of senior civil servant Sue Gray's report into partygate.

The report said Downing Street lockdown gatherings represented a "serious failure" and were "difficult to justify".

Mr Bell recalled how only 10 people were present at his grandmother's lockdown funeral before asking Mr Johnson: "Does the prime minister think I'm a fool?"

Letters continue to come in as exodus of aides hits Number 10

The trickle of letters comes amid the departure of five Number 10 aides within the space of 24 hours.

Number 10 Policy Unit member Elena Narozanski is the latest aide to leave Mr Johnson's Downing Street operation amid the continuing fallout from partygate.

Her departure comes after Number 10 revealed Dan Rosenfield, the prime minister's chief of staff, and Martin Reynolds, Mr Johnson's principal private secretary, are leaving their roles.

Two other key advisers to Mr Johnson - press chief Jack Doyle and policy chief Munira Mirza - have also resigned.

Downing Street has said the departures of Mr Rosenfield, Mr Reynolds and Mr Doyle - who had become embroiled in the partygate scandal - had been agreed before Ms Mirza's surprise resignation and the trio were departing by "mutual consent".

PM seeks to reassure backbenchers they matter

The PM continues to come under pressure over Downing Street gatherings during COVID restrictions in 2020 and 2021, 12 of which are being investigated by the Metropolitan Police.

Mr Johnson sent a letter to all Conservative MPs on Friday, seen by Sky News, saying he is "committed to improving the way 10 Downing Street, and government more broadly, works", adding that the process "is now under way".

The PM said he will be working with Sir Graham and heads of the 1922 Committee "to re-establish backbench policy committees" as he sought to reassure backbenchers their work and views matter to his government.