Christian Wakeford has become the latest Tory to submit a vote of no confidence in the Prime Minister after a series of Downing Street parties during lockdown.
The Bury South MP branded the scandal as 'embarrassing' in a series of damning comments last week.
Boris Johnson has been accused of lying to the House of Commons by claiming he thought that a "bring your own booze" garden party on May 20, 2020 was a work event.
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More than 20 Tory MPs who were elected at the 2019 election are expected to submit letters of no confidence in the PM, the Mirror reports.
One Tory MP was quoted as saying: "His time has gone."
Another said: "I think we’ve done it... The whips operation completely backfired. Colleagues now have a sense they aren’t alone."
Mr Wakeford became the seventh Tory MP to publicly announce that he had sent a letter of no confidence in the Prime Minister on Tuesday (January 18).
In a pointed message to Boris Johnson, he called for "openness, trust and honesty" in politics, and "that starts from the top."
The Bury South MP said on Twitter: "How do you defend the indefensible? You can’t!
"It’s embarrassing and what’s worse is it further erodes trust in politics when it’s already low.
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"We need openness, trust and honesty in our politics now more than ever and that starts from the top!"
Exactly 54 letters need to be submitted, with reports suggesting that around half that number had already been reached.
In another blow for Mr Johnson, Whitehall enforcer Sue Gray is expected to interview Dominic Cummings before presenting her findings into Downing Street parties.
A Government source told the Mirror: "Sue wants the inquiry to be as robust as possible. He is going to be interviewed by the time the inquiry concludes."
In explosive claims, Mr Cummings had claimed the PM "waved aside" concerns about the rule-busting gathering at the height of lockdown.
He claimed that he and "other eyewitnesses" would be willing to swear under oath that senior officials had warned the event was against the rules.
During a broadcast interview, Mr Johnson claimed that "nobody told me" the No 10 garden party was "against the rules".
This was even though his own Government drew up the rules which millions of other Brits had followed at the time.
The PM, on a visit to a North London hospital, repeatedly refused to rule out resigning if he was found to have misled Parliament.
"Well, let's see what the report says," he said.
