In a lengthy public statement, Harrogate and Knaresborough's MP Andrew Jones reiterated his long-held view that "lawmakers can’t be law breakers".
Mr Jones says his stance has been partly fuelled by what he said were the "many harrowing stories in emails I've received where people couldn’t visit elderly relatives or mourn them at their funerals."
Mr Jones, who has been critical of his party leader's attitude towards Partygate ever since the issue first blew up six months ago, added it wasn't merely a case of Boris Johnson breaking his own lockdown rules during the Covid pandemic, it was the repeated statements the PM had made at each step as the Partygate revelations grew.
Most people, Mr Jones said, "would have judged those statements to be untrue".
Vote of confidence in Boris Johnson as Leader of the Conservative Party.
The full statement from Andrew Jones MP, Harrogate & Knaresborough
"At the beginning of all the investigations into Partygate I said ‘lawmakers can’t be law breakers’.
"I meant it and that is, in part, why I will not be supporting the Prime Minister in tonight’s confidence vote.
"In part also it is because of the Prime Minister’s responses when questioned in the Commons and by the media about gatherings in Downing Street.
"I have received emails from hundreds of people and the overwhelming majority of these think the Prime Minister should resign.
"But it only takes one letter with one good reason for the point to be made.
"There were many harrowing stories in those emails where people couldn’t visit elderly relatives or mourn them at their funerals.
"These were people following the rules the Prime Minister set and championed.
"And I know, in the great scheme of global events, a fixed penalty notice for the impromptu presentation of a birthday cake does not seem much.
"But this isn’t a parking fine; it was a fine for a breach of rules brought in to protect public health and the NHS in a time of national crisis.
"But on the other occasions separate to the birthday cake incident, the Prime Minister must surely have noticed as he raised a glass to say farewell to a leaving member of his staff that the tables around him were groaning with bottles of wine.
"He must surely have questioned whether what was happening was within the rules.
"Most of us would have known at a glance that these gatherings were not work.
"A member of my team left during lockdown and we said goodbye remotely.
"Had I walked in on a gathering of the type at which we saw the Prime Minister photographed then I would have stopped that event.
"And in all the statements after that about the rules having been followed at all times the Prime Minister must have had a question mark in his mind over whether that was true because of what he had seen with his own eyes.
"Most would have thought better of making such definitive statements because most people would have known in their hearts that work meetings do not consist of party games and copious bottles of wine.
"So I believe that the Prime Minister, in addition to the fixed penalty notice, has shown a lack of judgement in making the many statements about Partygate that he did.
"Most people, having seen what he had seen, would have judged those statements to be untrue and, therefore, not made them.