Tory Gillian Keegan has reported a leaflet telling “national traitor” MPs who backed Theresa May’s Brexit deal “we’re watching you” to the police.
The Chichester MP, an ally of the PM, became concerned after the material - bearing UKIP’s party logo - was allegedly delivered to homes in her constituency.
Keegan, who voted for May’s deal and is Chichester’s first female MP, has reported the matter to Sussex Police.
The force has confirmed its officers are investigating an incident of malicious communications and appealed for people with information to get in touch.
Keegan told HuffPost UK: “The referendum was highly divisive and understandably people on both sides have strong feelings about the future direction of this country.
“However, difference of opinion is never an excuse for threatening or abusive behaviour.”
The alarming leaflet reads: “We would classify anyone prepared to vote for this agreement as a national traitor, or state enemy – and we’re watching you.”
UKIP has denied the leaflet was sanctioned at a national level and said it had launched an internal investigation.
The leaflet has been distributed, both in Sussex and outside the Houses of Parliament, amid fresh concern over the safety of MPs, particularly women.
A number of MPs and high-profile figures have been mobbed by so-called pro-Brexit and pro-Donald Trump campaigners near Westminster in recent weeks.
Tory MP Anna Soubry, who supports remaining in the EU, was followed down the street by one group and faced chants of “Nazi” while she gave an interview to reporters. The Broxtowe MP later revealed one of her staff was also targeted and called a “spineless c***” on his way into work.
Left-wing campaigner Owen Jones was also subjected to homophobic abuse and spat at after an interview to Sky News on College Green, which is opposite parliament.
The surge in threats and intimidation comes in the wake of the murder of Labour MP Jo Cox outside of her Yorkshire constituency office by a far-right activist.
The man convicted of her killing, Thomas Mair, was asked his identify in court, to which he replied: “My name is death to traitors, freedom for Britain.”
HuffPost UK learned that copies of the leaflet were produced and distributed outside Westminster the day of the so-called meaningful vote on May’s Brexit deal.
Sussex Police said: “We have received one report of a leaflet supposedly from UKIP since intense social media activity around mid-January.
“A woman from Chichester reported on January 30 that she had had a leaflet posted through her door on January 19.”
A spokesman for UKIP said: “The material in question was not sanctioned by UKIP Head Office, and as such is not an official communication of the national United Kingdom Independence Party.
“UKIP is investigating the source of this material.”
Anyone with information about the leaflet should contact Sussex Police at: www.sussex.police.uk/contact/