The former cabinet minister Dame Andrea Leadsom has blamed Boris Johnson for “unacceptable failings of leadership” over the partygate scandal.

The former business secretary is the 40th Conservative MP to have publicly voiced their displeasure with the prime minister since he was fined by the Met for breaching lockdown rules in Downing Street, according to a Sky News tally.

Dame Andrea sent a letter to her South Northamptonshire constituents on Monday, in which she said that after careful study of the Sue Gray report into the partygate scandal “it is painfully clear to me that given the extent and severity of rule-breaking taking place over a 20-month period, it is extremely unlikely that senior leaders were unaware of what was going on”.

The senior Brexiteer, who supported Mr Johnson in his leadership campaign, said: “I, therefore, agree with Sue Gray’s conclusions that there have been significant failures of leadership, both political and official, in No 10 and the Cabinet Office.

“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 added that she and all Conservative MPs “must now decide on what is the right course of action that will restore confidence in our government”.

A steady drip of Tory MPs voicing their displeasure with the prime minister after the release of the Sue Gray report last Wednesday has turned into more of a stream.

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Three more came forward on Monday, including Elliot Colburn, one of the 2019 intake, who confirmed he has sent a letter of no confidence in the prime minister to the chair of the 1922 Committee of Tory backbenchers.

This brings it to 28 Tory MPs who have publicly called for the PM to go over the partygate scandal.

Former Conservative leader Lord Hague reacted to Dame Andrea’s comments by telling Times Radio: “The fuse is getting closer to the dynamite here and it’s speeding up.”

He said the PM is in “real trouble” when senior Tories such as Dame Andrea and Sir Bob Neill come out against him.

Lord Hague said it looks like Mr Johnson could face a leadership vote “either next week or around the end of June”.

A total of 54 letters – 10% of Tory MPs – must be handed to Sir Graham Brady to trigger a leadership vote, and he is the only person who knows how many have been submitted.

Ahead of the threshold being met for a leadership vote in Theresa May, only 24 MPs had revealed they had submitted a letter, but 48 or more letters had actually been handed in.

Downing Street is likely preparing for a possible leadership vote, which is unlikely to happen this week as MPs are on recess.

Some MPs have said they will not reveal their opinions on the conclusion of the Sue Gray report until after recess, which has prompted speculation in Westminster that more will hand in letters next week.