David Warburton, 57, was suspended from the party in April 2022 pending the outcome of an Independent Complaints and Grievance Scheme (ICGS) investigation into the allegations.

He claimed he had been denied a fair hearing by the ICGS and prevented from "speaking out" while it probed the claims.

His departure means Prime Minister Rishi Sunak and the Conservative Party are facing a fourth by-election after Boris Johnson and two of his allies, Nadine Dorries and Nigel Adams, announced they were resigning just over a week ago.

In his resignation letter, Mr Warburton said the "malicious allegations" and "flawed" 14-month investigation had "inflicted unimaginable and intolerable destruction on my family and on me".

He said had been left with "no choice" but to provoke "the upheaval of a by-election", adding: "It is my hope that, in so doing, I can freely illuminate the methods of an oversight system not fit for purpose, so that friends and colleagues in the House can see the perverted process by which their own judgement may at any time be freighted."

He claimed the ICGS "prevented" him from speaking out and that it "moves at a cripplingly glacial pace towards a predetermined conclusion".

In an interview with The Mail on Sunday, Mr Warburton admitted to taking cocaine after drinking "tons of incredibly potent" Japanese whiskey, but denied claims that he harassed a female political aide in his Westminster flat.

A House of Commons spokesperson said they could not comment on an individual case, but said that parliament remains "absolutely committed" to the ICGS.

Authorities still believe the ICGS is fit for purpose - although the complexity of some of the cases can lead to length investigations.

A Conservative Party spokesperson said: "The IGCS is an independent body set up by parliament and the Conservative Party respects its processes.

"Mr Warburton had the whip withdrawn over these complaints last April and has not sat in parliament as a Conservative since."

Liberal Democrat deputy leader Daisy Cooper said: "Time after time the Conservatives have mired themselves in sleaze and scandal neglecting the issues that really matter to people. Then they decided it was okay to leave local people in this seat without any proper representation at all.

"This by-election will be a clear contest between the Conservatives and the Liberal Democrats, Labour finished third last time and are completely out of the race. The people of Somerton and Frome need a local champion and they can have that at the next election by choosing our local Liberal Democrat candidate Sarah Dyke."

Labour's national campaign co-ordinator Shabana Mahmood accused the prime minister of being "too weak to act himself".

"Yet again, we see a Tory MP resigning in disgrace, after Rishi Sunak was too weak to act himself. Enough is enough," she said.

"Britain can't afford this weak Conservative prime minister with a party too divided to govern as families struggle with the cost of living. It's time to turn the page on 13 years of Tory chaos. We need a general election now and a Labour government led by Keir Starmer."

Read more:
How is a by-election triggered - and why is resigning so complicated for MPs?

Mr Warburton was first elected in 2015 with a majority of 20,268, or 53% of the vote. According to the MP's website, at 18.3%, this represented the largest constituency swing to the Conservative Party.

He was re-elected in 2017 and 2019, with majorities of 22,906 and 19,213 respectively.