Outspoken Lee Anderson has called on Laurence Fox to stop talking "b******" in an ugly Twitter spat - hours before pulling out of the launch of a rebel Tory report over immigration.
The party's deputy chairman was expected to help launch a new plan accusing the Government of failing to live up to election promises.
But audience members were told he was too sick to deliver his speech, amid questions over whether Mr Sunak would have to step in if he put in an appearance.
Mr Anderson this morning got embroiled in a row with actor and musician Mr Fox, who now heads the right-wing Reclaim Party.
He told Mr Fox to "grow up" in a furious tweet posted just after 8am - five hours before his scheduled speech at the controversial New Conservatives event.
When Mr Fox asked why he hadn't voted on a motion on social transitioning - put forward by shamed ex-Tory MP Andrew Bridgen last week - Mr Anderson ranted: "Stop this ignorance.
"I was not there so could not vote for it. I support the motion but you really need to stop this b******s you are spouting, its beneath you.
"I have explained this before but you keep coming back to cause a pile on. I thought better of you. Grow up."
He then added in a follow-up: "I wasn't there because I knew nothing about it, as did many other MPs who would have supported it."
Mr Anderson was set to be the keynote speaker at the launch of the New Conservatives' plan to cut migration.
Twenty hardliners put their name to a list of demands as they try to fight off electoral wipeout - including cutting a scheme granting visas to foreign carers.
They also called for a crackdown on foreign students, prompting fears universities could be bankrupted if it was ever implemented.
The 12 demands also include a cap on the number of refugees permitted per year at 20,000 and a limit on social housing awarded to immigrants.
Danny Kruger, who heads the New Conservatives group, said Mr Anderson is "ill in bed" and said he "has been supportive" of the extreme immigration demands.
But he said loudmouth Mr Anderson was unable to put his name to the list of demands because of his role in the party hierarchy.
Downing Street refused to say whether Mr Sunak was frustrated by Mr Anderson's support for the New Conservatives group, saying there are "different views on both sides".
"I think on this issue as with many others there are different views on both sides. It is the Prime Minister's job and the Government's job to strike the right balance," the Prime Minister's official spokesman said.
It comes as backbenchers up the pressure on the PM to bring down net migration.
In a blistering attack in the Commons, Labour Shadow Home Secretary Yvette Cooper asked Suella Braverman: "Will the Home Secretary wish the deputy chairman of the Conservative Party a speedy recovery from the terrible sick bug which I understand has prevented him from launching this morning an entirely different Conservative immigration policy to the policy of the Conservative Home Secretary?
"And does she agree with him that social care visas should be cancelled - yes or no?"
Ms Braverman sidestepped the question and claimed Labour wants "open borders and unlimited migration".
But the Government faces uncomfortable questions from within their own ranks, with red wall Tory MP Miriam Cates telling the New Conservatives event: "We've not delivered on our promises and we must".
She warned of an "addition to cheap foreign labour", which she blamed for soaring immigration levels.
The MPs, led by Tom Hunt, are angry that he has failed to deliver on the 2019 Conservative manifesto pledge to slash the numbers of people arriving in this country.
Net immigration stood at 226,000 when Boris Johnson made the promise, but it has risen to an all time high of 606,000 since then.
Despite the very public launch of the campaign, supporters claimed they were not challenging Mr Sunak's authority.
Mr Hunt said: "I don't see this as undermining the Prime Minister at all. I see it as a helpful contribution to the debate."
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