Nigel Farage has been interviewed by Scotland Yard over claims that his alleged ‘ex-mistress’ fabricated a sexual assault case against a Tory MP, The Mail on Sunday can reveal.

The Ukip leader was quizzed by detectives investigating whether Annabelle Fuller falsely accused Andrew Bridgen of groping her on the balcony of his flat after an evening in a Westminster pub.

Mr Farage’s police interview, conducted earlier this month, came after officers gained access to Ms Fuller’s phone records to check her account of events with people she spoke to that night.

Nigel Farage, left, was questioned after police accessed Annabelle Fuller's phone records, right

Sources have told this newspaper the encounter between the police and Mr Farage was ‘lively’, with the politician asking forensic questions about the investigation.

‘It was debatable who was actually interviewing who,’ said one.

There is no suggestion Mr Farage was aware at any stage of any alleged wrongdoing by Ms Fuller. He was not cautioned before the interview.

When contacted last night, Mr Farage said: ‘I would expect any conversation I have had with the Metropolitan Police to be kept confidential. I am appalled that any detail has been disclosed.’

The astonishing saga started in June 2011 when Mr Bridgen, 50, was arrested after Ms Fuller claimed to have been touched inappropriately after returning to Mr Bridgen’s flat with the MP and a fellow drinker. She fled from the property in the early hours –grabbing Mr Bridgen’s BlackBerry mobile phone and Commons pass in the process – before filing her complaint.

Charges against Mr Bridgen were dropped when Ms Fuller, 33, withdrew her allegations. The MP for North West Leicestershire says the case wrecked his marriage to wife Jackie, and he now has limited contact with his children.

Ms Fuller falsely accused Tory MP Andrew Bridgen, left, of groping her following a night out in Westminster, an allegation which led to his arrest and the collapse of is marriage to is wife Jackie, right

Ms Fuller was described as Farage’s ‘former mistress’ under parliamentary privilege last year by former Ukip MEP Nikki Sinclaire. Both Mr Farage and Ms Fuller, who until last year worked as an aide to the Ukip leader, strongly deny an affair.

Last summer, Ms Fuller said she took an overdose and tried to cut her wrists after a confrontation with Mr Farage’s wife Kirsten, at a party celebrating Ukip’s success in the European elections.

Mr Farage has faced turmoil within his party since he backed out of his pledge to resign if he failed to win Thanet South at the Election. He did stand down as leader –but for just three days.

Last week, his deputy Suzanne Evans appeared to have been dumped as a party spokesman for saying that Mr Farage was a ‘very divisive’ figure who should not lead the EU ‘Out’ campaign. A day later she was ‘un-sacked’ as she had been removed without ‘proper authority’.

Scotland Yard’s investigation into Ms Fuller was launched after Jasna Badzak, a former Ukip official, said that following the incident Ms Fuller had boasted to her of seeing messages from David Cameron on Mr Bridgen’s BlackBerry.

Last year, Ms Badzak was sent a police ‘Prevention of Harassment’ letter for talking to The Mail on Sunday, on the grounds that it led to Ms Fuller being ‘subjected to numerous phone calls and emails’.

It prompted Tory MP Zac Goldsmith to write to London Mayor Boris Johnson urging him to look at whether police officers were abusing their position by trying to stop a newspaper investigation of Ukip.

Scotland Yard said: ‘We would never comment on who we interview as part of an investigation.

‘The inquiries are ongoing.’