Conservative MP Peter Bone faces a six week suspension from parliament, after being found guilty of “many varied acts of bullying” and “sexual misconduct”.
It means Rishi Sunak is facing another damaging by-election defeat.
An official Westminster watchdog said on Monday that Bone, 70, had committed the offences against a member of his staff in 2012 and 2013.
“The bullying involved violence, shouting and swearing, mocking, belittling and humiliating behaviour, and ostracism,” the Independent Expert Panel (IEP) said.
“This wilful pattern of bullying also included an unwanted incident of sexual misconduct, when the complainant was trapped in a room with the respondent in a hotel in Madrid.
“This was a deliberate and conscious abuse of power using a sexual mechanism: indecent exposure.”
Bone has denied the allegations and appealed the IEP’s initial decision, but it was dismissed.
In a statement today he said: “None of the misconduct allegations against me ever took place. They are false and untrue claims. They are without foundation.”
But if the Commons approves the suspension it could trigger a by-election in Bone’s Wellingborough constituency.
When an MP is suspended for 30-days or more it triggers a recall petition in their seat.
If 10% of registered voters in the constituency sign the petition, a by-election is held.
The Conservatives won Wellingborough with a 18,540 majority in 2019, but Labour and the Lib Dems have both overturned larger majorities at by-elections since then.
Bone, a prominent Brexiteer, has been the MP for Wellingborough since 2005 and briefly served in government as deputy leader of the Commons under Boris Johnson and Liz Truss.