Former British cabinet minister Andrea Leadsom has become the latest senior Tory to criticise Prime Minister Boris Johnson for his "unacceptable failings of leadership" over lockdown parties in Downing Street.

In a letter to her constituents, Ms Leadsom said the "extent and severity" of the rule-breaking in No 10 exposed in the Sue Gray report meant it was "extremely unlikely" the senior leadership did not know what was going on.

"The conclusion I have drawn from the Sue Gray report is that there have been unacceptable failings of leadership that cannot be tolerated and are the responsibility of the Prime Minister," she said.

In the message, which was shared on social media, she stopped short of directly calling on Mr Johnson to resign and did not say whether she had submitted a letter calling for a no confidence vote.

Andrea Leadsom

Ms Leadsom concluded however by saying: "Each of my Conservative colleagues and I must now decide individually on what is the right course of action that will restore confidence in our government."

Her intervention is a further blow to Mr Johnson as a steady stream of Tory MPs have been calling on the Prime Minister to stand down in the wake of Ms Gray's report last week.

A committed Brexiteer, Ms Leadsom backed Mr Johnson for the leadership in 2019 after pulling out of the contest herself, underlining the fact that discontent with the Prime Minister extends across the party.

Former foreign secretary William Hague said the Prime Minister is "in real trouble" and that Tory MPs are "moving towards having a ballot" on his leadership.

Allies of Mr Johnson had been hopeful he had escaped unscathed following a relatively muted initial response to Ms Gray's report last week, but Mr Hague said it is proving to be "one of those sort of slow-fuse explosions in politics".

"It's still going along. A lot of people misread it really, the events of last week as meaning the trouble is over, Boris is free and that's actually not the mood in the Conservative Party, which is very, very troubled about the contents of that report," he told Times Radio.

"So I think the Conservative Party will need to resolve this one way or another, obviously because to be an effective party they either need to rally behind the Prime Minister they've got, or they need to decide to force him out.

"I think they're moving towards either next week or around the end of June, they are moving towards having a ballot, it looks like that."

Former education secretary Justine Greening said Mr Johnson needs to "get a grip or get out", saying there is a "real jitteriness" among Conservative MPs.

Ms Greening, who was among 21 pro-Remain rebels thrown out of the party by Mr Johnson and is now no longer an MP, likened his position to that of Theresa May when she was under fire from Tory Brexiteers.

A steady stream of backbenchers have called on Mr Johnson to go after Ms Gray's report laid bare a drinking culture at the heart of the British government while raising renewed claims he misled parliament.

Under party rules, the chairman of the backbench 1922 Committee Graham Brady must call a vote of confidence in Mr Johnson's leadership if 54 Tory MPs - 15% of the parliamentary party - submit a letter calling for one.

So far, more than 25 MPs have publicly called on the Prime Minister to stand down - although not all of them have said whether they have written to Mr Brady.

However, it is also widely believed in Westminster that a number of others have put in letters without declaring their intentions amid speculation the tally is approaching the total needed to trigger a vote.

The arts minister, Stephen Graeme Parkinson, said it is "pointless" to speculate on numbers but acknowledged the continued uncertainty is a "distraction" as the government seeks to focus on the cost-of-living crisis.

"It's pointless speculating about something unless or until it happens," he told Sky News.

"It's a distraction from the work of government, and in government we're getting on with making sure that we grow the economy to help with the cost of living."

Rishi Sunak was fined by the Metropolitan Police over a gathering

The latest developments come as No 10 is under renewed pressure to say if Mr Johnson's wife hosted a second lockdown party in the Downing Street flat on the day of the Prime Minister's 56th birthday.

Earlier in the day on 19 June 2020, Mr Johnson was present at an impromptu gathering in the cabinet room, which led to him being fined by the Metropolitan Police along with his wife and Chancellor Rishi Sunak.

The government had already been facing questions over another event in the flat later in the year, on 13 November, when Mrs Johnson reportedly held the so-called "'Abba party' to celebrate the departure of Dominic Cummings in the fallout from a bitter No 10 power struggle.

In her report, Ms Gray said she had only gathered "limited" evidence on the event when she had to stop due to the police investigation, and that she did not consider it "appropriate or proportionate" to resume after officers concluded their inquiry.