A lesbian activist who founded a Tory gay group but defected to Labour has revealed how she had an affair with a married Tory councillor who also slept with a Tory MP.

Student Anastasia Beaumont-Bott hit the headlines earlier this year when she announced her defection to Labour after the then-shadow home secretary Chris Grayling said he thought bed and breakfast owners should be able to bar gay couples.

She had founded LGBTory but left the Conservative Party acrimoniously two years ago. In April, she announced she was joining Labour and appeared on a platform with former minister David Miliband to condemn homophobia.

Ms Beaumont-Bott, 21, said this weekend she had an affair with James Dinsdale, a 27-year-old Tory councillor, while he was sleeping with 37-year-old Tory MP Caroline Nokes, who is also married.

She said the experience was so bad it helped her become a lesbian.

She told the Daily Mail she met Mr Dinsdale in 2008 through Conservative Future, the party’s youth wing, and the pair began sleeping together.

She said: “It was only after about three weeks that I found out he was married, when a friend told me.

“It was bizarre to be going out with someone and then find out he was married. At the time, I thought I was bisexual, but have since become a lesbian. His behaviour probably did help underline that what I feel for men is nothing to what I feel for women.

“It wasn’t until I read in the papers about his four-year affair with Caroline Nokes that I realised he was seeing her at the same time as me – and his wife.”

Mr Dinsdale split from his wife Georgina last year.

Mrs Nokes, the newly elected MP for Romsey and Southampton North, was known as one of Tory leader David Cameron’s A-list female candidates, or the “Cameron Cuties”.

Also married, she had signed up to the Westminster Declaration, a Christian charter which proclaims signatories’ opposition to same-sex marriage, civil partnerships and adultery.

One passage says: “We pledge to support marriage – the lifelong covenantal union of one man and one woman as husband and wife. We believe it is divinely ordained, the only context for sexual intercourse, and the most important unit for sustaining the health, education, and welfare of all. We call on government to honour, promote and protect marriage and we refuse to submit to any edict forcing us to equate any other form of sexual partnership with marriage.”

Her affair with Mr Dinsdale was revealed when the Sunday Mirror published photographs of them secretly meeting at a London hotel.

Mrs Nokes reportedly discovered a hidden camera in her room, which has been handed to police. It is said to have filmed her and Mr Dinsdale having sex.