UKIP flyers saying anyone who votes for Theresa May's Brexit deal is a 'national traitor' have been sent to Sussex homes.
However UKIP head office said it had not sanctioned the flyer.
The flyer, which carries UK Independence Party (UKIP) branding, sets out why the party thinks MPs should vote against the Prime Minister's deal.
However there has been outcry at a notice written in capitals at the bottom of the leaflets. It says: "We would classify anyone prepared to vote for this agreement as a national traitor, or state enemy - and we're watching you."
There have been calls for the matter to be reported to the police since a photo of the leaflet was shared on social media.
After seeing the posts on social media, a Chichester resident got in touch to say that they had received the leaflet in question.
A spokesman from UKIP head office 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.”
The regional branch of UKIP has been approached for comment