Backbencher Andrew Bridgen has said he is suing Matt Hancock after the ex-Health Secretary condemned him for comments about Covid vaccines.
Mr Bridgen was stripped of the Tory whip after appearing to compare the effects of coronavirus jabs to the Holocaust.
Amid a furore over the comments, Mr Hancock, who lead the Department of Health during the pandemic, raised the issue during Prime Minister's Questions.
He also restated his criticism on Twitter, which is not covered by parliamentary privilege. This protects MPs from being sued for defamation for speeches in the Commons.
Mr Bridgen had already threatened legal action against Mr Hancock in relation to the tweet.
The North West Leicestershire MP tweeting that he would "allow Matt three days to apologise publicy [sic] for calling me an antisemite and racist or he will be contacted by my legal team".
Mr Bridgen has said that he is not antisemitic while also defending his language about the safety of coronavirus vaccines, claiming that his suspension from the Conservative parliamentary party "says much about the current state of our democracy".
In the days after the original remarks, he also said he was not racist and was "speaking to a legal team who will commence action against those who have led the call suggesting that I am".
Mr Bridgen tweeted today: "I can confirm that Matt Hancock had a legal letter before action from Bad Law Team on my behalf regarding defamation on Monday."
The legal action is being supported by the Reclaim Party and the Bad Law Project, which is linked to Reclaim Party leader Laurence Fox, claims to oppose "political ideology disguised as law".
A spokesperson for Mr Hancock said: "What Matt said was obviously not libellous and he stands by his comments.
"Rather than wasting his time and money on an absurd libel case he will undoubtedly lose, let's hope Bridgen does the right thing and apologises for the hurt he's caused and keeps his offensive view to himself in future."
