The prime minister is alleged to have referred to the MP as “Pincher by name, pincher by nature” before making him deputy chief whip in February.
The MP for Tamworth in Staffordshire resigned from the role after being accused of drunkenly groping two men in a private members’ club in London.
It was the second time he resigned from the whips’ office after Conservative candidate Alex Story accused him of making an inappropriate advance in 2017.
The latest in a line of sexual misconduct allegations featuring Tory MPs was adding to Mr Johnson’s struggle to regain his authority after the partygate scandal.
There were suggestions his handling of Mr Pincher will embolden Conservative rebels’ attempts to change the rules of the 1922 Committee of backbenchers so they can hold another no confidence vote within the next year.
Critics of the PM told The Times his handling of the scandal “had sharpened minds to act” and revitalised the campaign to oust him Downing Street.
A stream of fresh allegations against Mr Pincher emerged over the weekend, as one Tory backbencher said claims about his behaviour had been “swirling around Westminster for years”.
“Only Boris Johnson could have looked at this guy’s record and thought ‘he deserves a promotion’,” she added in a statement.
“This prime minister is clearly happy to sweep sexual misconduct under the carpet in order to save his own skin.”
She also questioned why the Tory whip was not suspended, meaning the MP now sits as an independent, until Friday when the incident took place at the Carlton Club on Wednesday.
A Downing Street source has argued that Mr Johnson took the move after speaking to a Tory MP who was with one of the men allegedly groped by Mr Pincher.
“The account given was sufficiently disturbing to make the PM feel more troubled by all this,” the source told the PA news agency.
Dominic Cummings, the prime minister’s former chief aide turned critic-in-chief, said Mr Johnson referred to the MP “laughingly in No 10 as ‘Pincher by name, pincher by nature’ long before appointing him”.
Downing Street has not disputed that account, but Work and Pensions Secretary Therese Coffey argued that Mr Johnson did not know “specific claims” about the MP.
“I don’t believe he was aware, that’s what I’ve been told today,” she told Sky’s Sophy Ridge on Sunday.
Liberal Democrat chief whip Wendy Chamberlain laid the blame for the “sleazy toxic Government” with Mr Johnson.
“He must now be forced to reveal what he knew before making the appointment,” she added.
Current party rules dictate that another challenge cannot be held for a year after last month’s confidence vote, which Mr Johnson won despite 148 Tory MPs voting against him.
But amid renewed questions over his judgment, there are suggestions that the upcoming elections for the 1922 Committee’s executive could focus on a compromise proposal to allow a vote sooner.
The change would mean a second confidence vote could be held if 25% of Tories in the Commons – 90 MPs – submit letters to the 1922 leadership, double the current amount required to trigger an initial vote.
Under investigation by Parliament’s Independent Complaints and Grievance Scheme, Mr Pincher said he is seeking “professional medical support” and hopes to return to represent his constituents “as soon as possible”.
Meanwhile, a man told the Sun that Mr Pincher said “inappropriate things” before touching the top of his thigh in a meeting in the MP’s constituency office in 2018.
The Mail on Sunday alleged Mr Pincher threatened to report a parliamentary researcher to her boss after she tried to stop his “lecherous” advances to a young man at a Conservative Party conference.
The Sunday Times alleged he made unwanted passes at two Conservative MPs in 2017 and 2018 – after his first resignation as a whip.
A Tory MP told the Independent he was groped on two occasions by Mr Pincher, first in December 2021 and again last month.
In 2017, at the time a young Tory activist, Mr Story described Mr Pincher untucking the back of his shirt, massaging his neck and whispering “You’ll go far in the Tory Party”.
The new allegations reignited concerns about standards in Westminster after a string of Conservative MPs faced sexual misconduct claims.
In May, Neil Parish quit as MP for Tiverton and Honiton after admitting viewing pornography in the Commons chamber.
A month earlier, then-Wakefield MP Imran Ahmad Khan was jailed for 18 months for sexually assaulting a 15-year-old boy.
The Conservatives lost both by-elections that followed.
A third unnamed Tory MP has been told by whips to stay away from Parliament after being arrested on suspicion of rape and other offences.
In a statement, Mr Pincher said he would “co-operate fully” with the investigation.
“As I told the prime minister, I drank far too much on Wednesday night, embarrassing myself and others, and I am truly sorry for the upset I caused,” he continued.
“The stresses of the last few days, coming on top of those over the last several months, have made me accept that I will benefit from professional medical support.
“I am in the process of seeking that now, and I hope to be able to return to my constituency duties as soon as possible.”