A Ukip candidate has quit the race to contest a key seat at next year’s UK general election after making sexual harassment allegations against the populist party’s general secretary.

Natasha Bolter, a former Labour supporter and rising star in Ukip who addressed the party’s autumn conference, has pulled out of the hustings to run for the seat of South Basildon, in Essex.

Roger Bird, who has been Ukip general secretary since July, told the Guardian there were no criminal allegations involved.

It is expected that the former Conservative MP Neil Hamilton, once in line to be Ukip candidate for Boston and Skegness, is in a strong position to win the party’s Basildon nomination on Wednesday evening. Hamilton pulled out of the Boston race a month ago, leaving the nomination to be taken by a local candidate.

Bolter told the Times that Bird, who was involved in vetting candidates, made unwanted advances to her on the day she was interviewed after taking her to the Oxford and Cambridge Club in London. She claimed that on a second occasion he called her for a meeting and bought her an expensive dress to wear to the same club before telling her that she “now looked like a girl who could get in a taxi”.

She told the newspaper that he made further unwanted advances after a meal, which she again rejected. After this incident, Bolter claims to have received anonymous emails saying she had slept with Bird and was “bullying” her, at which point she made a complaint to party officials.

Bird claims the two had a sexual relationship, which Bolter denies. Bird told the BBC: “We were in a relationship briefly, but that relationship developed well after she had been admitted to the approved candidates list, so her selection was not connected to this.”

He added: “Natasha Bolter’s candidate assessment was conducted entirely within the rules, as the party has already verified. Subsequent to that, Natasha Bolter and I were in a consensual relationship. I am not the head of candidates and I do not dictate which candidates get selected for which seats, nor would I try to, although I have advised and helped a number of candidates over the course of my years in the party.

“The party quite rightly has a duty to investigate complaints of a potentially serious nature and I feel they have acted entirely properly in doing that … It is important that the party has instigated a formal process and I hope that will soon be brought to a conclusion.”

Ukip earlier released a statement saying Bird had been suspended, and declining to go into details. It said: “Shortly after a claim came to light about the conduct of Mr Bird with regard to candidate selection, the UK Independence party took action swiftly and decisively, including steps involving external human resources consultants, as well as following due process and the party’s constitution to the letter.

“Unfortunately, Ukip has had to suspend Mr Bird pending a full investigation into allegations made against him. The party has acted swiftly and decisively and will not tolerate impropriety of any kind amongst its staff.”

The party said it had received a report from HR consultants at the weekend and acted quickly thereafter. The party’s disciplinary committee will decide before Christmas on what action to take.

Within Ukip’s membership there is a concern, but also some suspicion that a former Labour party member has joined the party recently only to make such serious claims so soon. Bolter was a high-profile recruit from the Tower Hamlets Labour party and made a star appearance at the party’s autumn conference where she was introduced by Bird as a fellow Oxford PPE graduate

She said then: “Ukip is a fair party, respecting all nationalities and genders. We are an equal party. We have women at centre-stage, just look how many MEPs we have. The women in Ukip have flourished equally alongside our male counterparts. We are seen for our intelligence and aptitude. We are seen for our hard graft. We are respected for our contribution. I want to be a candidate for Ukip and I want to campaign to win but I want to be selected on merit.”

She has now told the Times that she encountered racism in every Ukip branch she visited, as well as regular sexism.

Bird previously held the role of chairman of Ukip in south-east England and was selected last month as a parliamentary candidate for the seat of Cities of London and Westminster.

The party has recently been under scrutiny over the way some local candidates have been deselected potentially to make way for bigger names. Bird was involved in at least one of these controversies, when he told the Portsmouth News that the removal of Douglas Denny in Portsmouth South was a “routine event”.

His LinkedIn page says he has been general secretary of the party since July and was previously a finance director and auditor.