The Leader of the House of Commons, Andrea Leadsom, who was a prominent figure in the Brexit Leave campaign, said British politics had “hit a new low” after receiving the letter, signed by “The Real 48 Per Cent” which threatened: “We’re coming for you”.
Police confirmed they were investigating four such messages sent to MPs. The letter read: “You have taken lives on our side. Now we will take lives on yours.”
Posting an image of the letter on Twitter, which was addressed to her in Parliament, the Cabinet Minister commented: “Pretty despicable whoever sent me this.
“We live in a democracy – death threats because you don’t agree? And unsigned? Coward…”
Ms Leadsom said many people were scared off social media by “internet trolls spouting racist and sexist abuse”, while campaign posters had been daubed with swastikas and protesters turned up “to hurl insults at speakers they disagree with”.
Referring to murdered Labour MP Jo Cox, who was shot and stabbed by white supremacist Thomas Mair in 2016, she said: “Passions ran high during the referendum campaign, but that is no excuse for the violent or hostile behaviour we have witnessed.
“Last week I was sent a death threat, signed by The Real 48 Percent – and I know that my fantastic colleague Anna Soubry, who campaigned for Remain, has also received death threats.
“This cannot be the new normal. We cannot stand by as some members of our society are silenced by bullying and scaremongering, and I include everyone in that – Remainers or Leavers, men or women, those on the Left or on the Right.
“Everyone’s voice matters, and we are calling out this unacceptable assault on our democracy.”
It comes after Prime Minister Theresa May called for a new offence in electoral law of intimidating parliamentary candidates and their campaigners.
A report by the Committee on Standards in Public Life recommended that internet giants such as Twitter, Facebook and Google be forced to reveal the percentage of reported offensive content that is taken down and how long it took to do this.
Earlier this month, Ms Leadsom published Westminster’s new independent grievance and complaints procedure in the wake of the “Pestminster” sexual harassment scandal.