Jacob Rees-Mogg has apologised to an NHS consultant for comparing him to a disgraced anti-vaxxer in a row over no-deal Brexit planning.
Mr Rees-Mogg likened Dr David Nicholl, a neurologist, to Andrew Wakefield, who started a bogus scare about the measles, mumps and rubella (MMR) vaccine and was subsequently struck off the medical register.
Dr Nicholl told Sky News the comparison "really beggars belief".
He also described Mr Rees-Mogg as a "coward" and challenged him to repeat his comments outside the House of Commons, where he would be without the protection of parliamentary privilege, the rule that anything said within the chamber cannot be challenged in law.
"If he does I'll sue him," Dr Nicholl said.
Leader of the Commons Mr Rees-Mogg said: "I apologise to Dr Nicholl for the comparison with Dr Wakefield.
"I have the utmost respect for all of the country's hardworking medical professionals and the work they do in caring for the people of this country.
"The government is working closely with the NHS, industry and distributors to help ensure the supply of medicine and medical products remains uninterrupted once we leave the EU on October 31, whatever the circumstances."
Health Secretary Matt Hancock said he was "glad Jacob has apologised".
He added: "It's vital clinicians can provide expert advice. I defend to the hilt the right of clinicians and civil servants to provide advice without fear or favour."
The apology came after Professor Dame Sally Davies, the chief medical officer, wrote to Mr Rees-Mogg to "express my sincere disappointment in the disrespectful way you spoke to and about Dr David Nicholl".
She added that comparing Dr Nicholl, whom she described as an "established medical expert", to Mr Wakefield was "frankly unacceptable".
The two men first clashed during a radio phone-in show, after Dr Nicholl asked Mr Rees-Mogg what mortality rate he would accept if the UK left the EU without a withdrawal agreement.
In response, Mr Rees-Mogg accused Dr Nicholl of "fear-mongering" and indulging in the "worst excess of Project Fear".
"I don't think there's any reason to suppose that a no-deal Brexit should lead to a mortality rate," he added.
Dr Nicholl was involved in the government's Operation Yellowhammer report into the impact of a no-deal Brexit.
Mr Rees-Mogg returned to the row in the Commons on Thursday. Addressing MPs, he compared Dr Nicholl to Mr Wakefield.
Mr Wakefield sparked controversy over the MMR vaccine after conducting research into a supposed link between it and autism.
There has since been found to be no link between the MMR jab and autism, with the General Medical Council finding that Mr Wakefield acted "dishonestly and irresponsibly".
Referring to Dr Nicholl's comments on a no-deal Brexit, Mr Rees-Mogg told MPs on Thursday: "Preparations are in place and they are being done with remarkable efficiency.
"But yes, a lot of Remainers wish to make our skins crawl.
"What he had to say, I will repeat it, is as irresponsible as Dr Wakefield in threatening that people will die because we leave the EU. What level of irresponsibility was that?
"And I'm afraid it seems to me that Dr David Nicholl is as irresponsible as Dr Wakefield."
Dr Nicholl told Sky News that Mr Rees-Mogg's comments in the Commons were "appalling".
He also described them as a "threat not just to the NHS but any whistleblower - any civil servant who's concerned for matters of safety".