A Westminster panel that oversees complaints against MPs has ordered a reinvestigation of sexual misconduct claims against David Warburton, who quit as an MP after a probe into his conduct.

The former Somerton and Frome MP was found guilty of three sexual misconduct allegations by the Parliamentary Commissioner for Standards, instead of the two upheld by the initial inquiry, the watchdog revealed as it called for both decisions to be reassessed.

Mr Warburton announced his resignation last month, hitting out at the “cripplingly glacial pace” of the investigation, which had already taken 14 months.

The Independent Expert Panel (IEP) on Tuesday upheld an appeal by Mr Warburton and ordered a fresh inquiry as it criticised the “materially flawed” original probe.

In a report, the panel said the initial investigation under Parliament’s Independent Complaints and Grievance Scheme (ICGS) recommended upholding two sexual misconduct allegations.

It came after a staffer in Mr Warburton’s Westminster office lodged a complaint with the ICGS in March 2022, making four sexual misconduct and two bullying and harassment allegations against him.

However, Parliamentary Commissioner for Standards Daniel Greenberg then reviewed the evidence and found Mr Warburton guilty of a third, unspecified sexual misconduct charge.

Mr Warburton appealed against Mr Greenberg’s decision on the grounds it was “procedurally flawed and unreasonable” and based on a “materially flawed” investigation.

He said the woman “had colluded with witnesses to fabricate” the “malicious” complaint against him, and that the ICGS failed to consider material he submitted to support his claim, according to the IEP’s report.

The panel said it made no findings on the allegations against Mr Warburton or his claims of fabrication, but ruled that a reinvestigation is “necessary and proportionate”.

Mr Warburton said: “I do very much welcome the IEP ruling that has upheld my appeal.”

However, the Liberal Democrats said he was “never fit to be an MP”.

“This whole process is dragging on indefinitely, while David Warburton’s alleged victims have been left without answers and his constituents without an MP,” Lib Dem deputy leader Daisy Cooper said.

“This Conservative government is mired in constant sleaze and scandal while people struggle to afford their mortgage or see their GP.”

Mr Warburton has admitted cocaine use, saying he took the drug after drinking “tons of incredibly potent” Japanese whiskey, but denied claims he harassed a female political aide in his Westminster flat.

When he announced his exit from Parliament in June, the former Tory MP hit out at the ICGS investigation, claiming he had been denied a fair hearing.

In his resignation letter 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 judgment may at any time be freighted.”

Mr Warburton’s move means Prime Minister Rishi Sunak faces another potentially difficult by-election in the Somerton and Frome seat.

The vote will take place on July 20, on the same day as the by-elections in Boris Johnson’s former seat of Uxbridge and South Ruislip, and Selby and Ainsty.