Tory leader Douglas Ross says Boris Johnson must resign if found to have attended a lockdown-breaking party.
However, ITV News knows he was there at a meeting held in his Number 10 back garden during the UK's first lockdown.
A Number 10 email inviting over 100 staff members was obtained to ITV News, substantiating claims made by his former senior assistant Dominic Cummings that a party was hosted there on May 20, 2020.
Mr Ross said the PM would have to quit if an investigation found he had attended the May party despite initially telling MPs he had not.
"I've always been clear that if you mislead Parliament, there's no defence and you have to leave," Mr Ross added.
“It's "If he violates the law by attending this party, he faces serious charges… I would not support the PM if he breached the law and went to that party."
The PM has not responded directly to the fresh claims and evaded an urgent question from Labour on Tuesday.
Prime Minister David Cameron refused to speak because of an ongoing inquiry into wider rule violation in Downing Street by top civil servant Sue Gray, angering MPs.
The PM "retains the confidence of the people of this country", Mr Ellis said if proven guilty of breaking the law.
Tories' John Caudwell told ITV News that if the PM was proved to have been at the drinks party, he would be "in trouble."
The Phones4U entrepreneur, who gave the Conservatives £500,000 ahead of the 2019 general election, said the party's recent problems had made him rethink his donation.
"The Tories will have to do far better than they're doing now for me to consider financially backing them," he stated.
"They made the rules and by golly if you made the rules you've got to stick to them," he said, without urging the PM to step down.
Boris Johnson sent a minister to answer Labour's urgent question.
But Conservatives in the Commons were unsupportive, with the government front bench nearly vacant, a "unusual" position.
Tory MP Nigel Mills questioned Mr Johnson's job security if he was discovered to have violated his own lockdown guidelines.
"I can't see how someone who hosted or attended a party can set Covid policy," remarked Amber Valley's MP.
"How can someone keep a position if he knew it was a party and chose to attend?"
Asked about the purported Downing Street party in the House of Commons, Mr Johnson responded he "should be here," according to Labour MP Afzal Khan.
"My mother died in March 2020. She died alone in hospital while I sat outside in my car, wanting to get close to her. Despite our pain, my family followed the rules.
We had our first Eid without my wonderful mum just three days after the Downing Street bash," she stated.
"It's fairly evident it's not the prime minister so we don't need to keep asking that question," Sir Lindsay Hoyle said, imploring opposition MPs to be quiet.
Jim Shannon, DUP MP, was overtaken with emotion while speaking about his mother-in-death law's in the Commons.
Strangford MP said: "Last week, Covid caused 3,000 deaths in Northern Ireland. My mother-in-law died alone."
Mr Shannon paused, then sat back down.
Hollywood icons have also reacted to the controversy. The comedian said he understands the "palpable" public outrage.
"It's understandable. Everyone followed the rules. I've always been naive, but now I wonder why we believe them.
"Of course we shouldn't trust the bosses!" he said.
The party was disclosed by ITV News after a leaked email from Martin Reynolds, the prime minister's primary private secretary, inviting Downing Street staff to the evening gathering.
Mr Reynolds advised them to "enjoy the great weather" despite England's severe coronavirus laws prohibiting social gatherings outside at the time.
"All that, as you know, is the subject of a full investigation by Sue Gray," Prime Minister Johnson said in response to the latest claim.
A leaked video by ITV News last year showed Number 10 personnel joking about going to a separate party, and the PM's silence this time raises doubts.