The chair of a rightwing thinktank due to give evidence to the Tory bullying inquiry has claimed the BBC was complicit in an attempt by the Conservative party to assassinate his character.

In a formal letter of complaint to the BBC Trust and the corporation’s director general, Tony Hall, Ben Harris-Quinney explains he was invited to appear on the Daily Politics programme to discuss vote-swapping between Tory and Ukip voters.

But he claims he was instead placed in a cubicle and subjected to a series of questions about his character, ultimately being accused by the presenter, Andrew Neil, of being a “Walter Mitty figure”.

Harris-Quinney, who is chairman of the Bow Group, the UK’s oldest conservative thinktank, alleges the Daily Politics took part in an attempted “political assassination” led by figures from Conservative campaign headquarters (CCHQ) including Paul Abbott, former of chief of staff to ex-party chairman Grant Shapps, and Mark Clarke, the former election aide at the heart of the bullying scandal.

In his letter of complaint, Harris-Quinney writes: “I am dealing with the Conservative party separately, my case is subject to several pending investigations into their practices and behaviour, much of which has now been exposed in the press.

“I believe, however, that there are very serious questions for the BBC to answer as to the complicity of the Daily Politics show in a politically motivated attempt to silence my voice and publicly defame my character.”

The BBC denied that the interview or questions were biased. Clarke declined to comment. Abbott was approached by the Guardian for comment but has not responded.

Harris-Quinney claims that before his appearance on the Daily Politics on 4 May last year Abbott, who no longer works for Shapps, sent a text message to friends and colleagues telling them to “watch this drive-by shooting”.

Facebook Twitter Pinterest Mark Clarke, the former Tory election aide at the centre of the Elliott Johnson bullying claims. Photograph: Joe Giddens/PA

He also claims in the letter that Clarke, whom Tory activist Elliott Johnson accused of being a bully before he apparently killed himself, was circulating messages about Harris-Quinney’s membership of the Conservative party before the interview was aired. Clarke then went on to attack Harris-Quinney on Twitter before, during and after the programme.

For the past nine months, Harris-Quinney, a critic of Tory centralisation, has been making enquiries into allegations of bullying and corruption within CCHQ, which came to the fore in September when the Guardian revealed Johnson, 21, had been found dead after claiming he was being bullied.

In the Daily Politics interview, Neil began by questioning Harris-Quinney on the vote-swapping issue, which he defended by explaining he supported conservative principles and voting Ukip would help bolster the Tories’ chances of forming a government.

But Neil, the former editor of the Sunday Times and current chairman of the Press Holdings group, which owns the conservative magazine the Spectator, then accused Harris-Quinney of not representing the Bow Group and just speaking for himself.

Neil questioned his guest’s credentials, challenging his position with the Madrid branch of Conservatives Abroad, which Harris-Quinney founded, at which point the interview became heated. Harris-Quinney asked: “What on earth has this got to do with the general election coming up? No one is interested in this.”

Neil replied: “I’m trying to find out who you represent other than Ben Harris-Quinney? Isn’t it true you’re a bit of a Walter Mitty character?”

When Harris-Quinney challenged Neil’s motivation, the presenter said: “That’s how it works, I ask the questions and you’re meant to answer them.”

Michael Heseltine, a patron of the Bow Group who appeared on the same episode of the Daily Politics, issued a statement rejecting Harris-Quinney’s suggestion that Tory supporters should vote Ukip. The Tory peer is not accused of being complicit in the alleged character assassination.

Turning to Lord Heseltine, whom Harris-Quinney understood was on the show to take part in the vote-swapping debate, Neil asked him for his view of the interview. “Well I think you destroyed him,” Heseltine said. “Who the hell is this?” He added: “This guy is of no account.”

The interview was covered by other media, including the Spectator, in which Harris-Quinney was characterised negatively. The Bow Group chairman said the experience caused him stress.

In his letter of complaint, he says: “This sort of politically motivated, personal, smear-based journalism does great damage to people’s lives and reputations, it is designed to cause maximum distress and harass the victims, and it has no place at the BBC.”

A BBC spokeswoman said: “We received this complaint yesterday, nine months after the broadcast in question. We will respond in due course, but categorically refute the suggestion that the interview or questions were based on anything other than the programme’s own editorial decisions.

“The credibility of Mr Harris-Quinney and his Conservative party credentials was key in assessing how significant the Bow Group’s report was. The BBC is impartial and all our journalists adhere to our editorial standards.”

BBC Daily Politics was accused of political bias last month when the shadow foreign affairs minister Stephen Doughty resigned live on the programme, leading to claims the broadcaster had orchestrated the move.

The BBC came under fire after it emerged that the corporation had deleted a blogpost from its website saying that the political editor of BBC News, Laura Kuenssberg, had “sealed the deal” for Doughty to resign live on air.

The timing of Doughty’s announcement was a blow to the Labour leader, Jeremy Corbyn, who was caught off guard during prime minister’s questions less than an hour later.

The Conservative party has appointed the law firm Clifford Chance to run an inquiry into bullying allegations that engulfed the party, but this has been rejected by Johnson’s parents and a number of other alleged victims.

Clarke was appointed by Shapps as director of RoadTrip2015, which bussed hundreds of young people into marginal constituencies in the run-up to the general election. Shapps has since resigned from a junior ministerial position over his role in the affair. Clarke has previously denied all allegations of bullying and other wrongdoing.