Former Tory chief whip Mark Harper has claimed "a number" of his Tory colleagues are considering submitting a letter of no confidence to the 1922 Committee.
Mr Harper is the latest Tory MP to break cover and declare the Prime Minister "unworthy" of the office he holds.
On Tuesday, he criticised Mr Johnson moments after the PM apologised 89 times in Parliament over his first Partygate fine.
“I regret to say that we have a Prime Minister who broke the laws that he told the country they had to follow," Mr Harper told MPs.
"[He] hasn’t been straightforward about it and is now going to ask the decent men and women on these benches to defend what I think is indefensible."
Today the senior Tory insists none of his colleagues have said he was "completely out of order" for voicing his lack of confidence in the PM.
Instead, Mr Harper claims a "small number" of Tory MPs "have said they're thinking very seriously about putting a letter in."
"Different colleagues will have different tests for this," Mr Harper told Times Radio.
"Some will want to wait until the police have concluded their investigations into all of the gatherings that the Prime Minister attended.
"Some will want to wait for that and for Sue Gray's full report to be published. And that's absolutely fine. Different colleagues will want to wait for different things."
While only just over a dozen MPs are calling on him to resign, that number could surge if his response to the fine goes down badly.
It takes 54 letters of no confidence to the backbench 1922 Committee to prompt a vote of no-confidence in the leader.
Tory MP David Simmonds - who represents a seat next door to the PM’s - did not call for him to quit but asked how he would “restore the moral authority of this government”.
But other Tory MPs spoke out in Mr Johnson's favour - with Steve Baker saying he had repented and should be forgiven, while Sir Edward Leigh said the only leader who should be removed was Vladimir Putin.
And Tory MP David Morris faced shouts of opposition as he gave a gushing tribute to his "trust" in the party leader, adding: “This Prime Minister is leading the world against Putin”.
The PM apologised after he had become the first ever law-breaking Prime Minister. But he denied he had deliberately misled MPs when he said "all guidance was followed completely" in No10.
Mr Johnson told Parliament: "Let me also say not by way of mitigation or excuse - but purely because it explains my previous words in this House - that it did not occur to me then or subsequently that a gathering in the Cabinet room just before a vital meeting on Covid strategy could amount to a breach of the rules."
