A former Cabinet minister who endorsed Boris Johnson to become Tory leader has slammed the "unacceptable failings" following the publication of the Sue Gray report.
Andrea Leadsom, who has twice run to become leader of the Conservative party, hit out at the Prime Minister as pressure mounts on him to resign over partygate.
Despite the criticism of Johnson the MP for South Northamptonshire stopped short of calling for the PM to quit.
In a letter to her constituents, published on social media, Leadsom said she believed it is "extremely unlikely that senior leaders were unaware of what was going on."
She wrote: "On studying the report carefully, however, it is painfully clear to me that given the extent and severity of rule breaking taking place over a 20 month period it it extremely unlikely that senior leaders were unaware of what was going on.
"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.
"I believe in the importance of high standards of integrity, and the Member of Parliament must strive to be good role models, very specifically because we are the lawmakers and therefore in a trusted position of power and authority.
"I am determined to be clear about my views as a matter of personal integrity - and the conclusion I have drawn from the Sue Gray report is that there have been unacceptable failings of leadership that cannot be tolerated and area the responsibility of the Prime Minister.
"Each of my Conservative MP colleagues and I must now decide individually on what the right course of action that will restore confidence in our Government."
Leadsom's criticism comes after four Tory MPs called for Johnson to step down on Monday.
It believed the number of Tories calling for the Prime Minister to quit could be in the "high 40s", according to the BBC.
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