Ukip leader Henry Bolton has started a relationship with a younger woman after splitting up with his wife.
Mr Bolton, 54, wrote to supporters to warn them to expect media coverage of a "change in my relationship status", but denied that it amounted to a "clandestine affair".
It is understood that he has been seeing Ukip supporter Jo Marney, 25, who describes herself in her Twitter profile as a "presenter, music journalist, model, actor, Brexiteer".
Sources close to Mr Bolton insisted that the relationship did not begin until after his split from wife Tatiana, 42, who gave birth to their second daughter at London's St Pancras Station in 2016 after going into labour on a train.
In a message to supporters on Wednesday, Mr Bolton said he expected his new relationship to be made public in The Sun newspaper. And a photo of him with Ms Marney was also published on the front page of the Daily Mail.
Mr Bolton, who was elected Ukip leader last September, said the story would "confirm that in recent weeks I've had a change in my relationship status", adding that it "might also suggest (inaccurately) that this change has involved a clandestine affair with a young lady who happens also to be a member of Ukip".
He said he had already made clear on social media that he had recently been spending time "with somebody who has become increasingly important to me", adding: "This isn't something that I've hidden."
He said: "I am in a new relationship and the media have latched onto that. However, whilst I fully accept that as a national political figure I can expect to be the subject of media attention, I also believe we're all entitled to a certain degree of privacy.
"As such, I'll be making no further comment on the matter."
Ms Marney has posted photographs of herself celebrating Christmas Day and Boxing Day with Mr Bolton, while he wrote on Twitter on New Year's Day: "Myself and @Jo_Marney nearly run over by George Osborne in Covent Garden".
Mr Bolton urged Ukip supporters not to allow the revelations to distract him or the party from their "vital mission", promising a "significant ramping up" of political activity in the coming three months.