Boris Johnson’s decision to make Tory MP Peter Bone a minister despite an ongoing inquiry into allegations of sexual misconduct must be formally investigated, the country’s most powerful official has been told.

Bone faces a six-week suspension from the Commons and losing his seat in a by-election after a Westminster ethics watchdog found he bullied and sexually harassed a member of his staff.

The Lib Dems have demanded Simon Case, the cabinet secretary, access Johnson and Liz Truss’ emails and text messages about the Wellingborough MP to avoid a “Conservative Party cover up”.

In one incident, Bone “trapped” a young male employee in a Madrid hotel room and indecently exposed himself.

Bone was found by the independent expert panel inquiry to have “verbally belittled, ridiculed, abused and humiliated” the man, who was in his early 20s.

He has denied the allegations. In a statement Bone said: “None of the misconduct allegations against me ever took place. They are false and untrue claims. They are without foundation.”

A complaint was first raised about the behaviour in 2015 when David Cameron was prime minister and again in 2017 when Theresa May was in No.10. The Conservative Party opened an investigation in 2018.

Bone, who was first elected in 2005, has spent the majority of his career as a backbencher.

But in July 2022, Johnson appointed Bone to the job of deputy Commons leader. It involved handling how complaints of bullying in parliament are dealt with.

When Liz Truss’ became PM, she sacked Bone and abolished the role entirely.

Christine Jardine, the Lib Dem Cabinet Office spokesperson, said on Tuesday: “We need an urgent inquiry to understand what Conservative ministers knew about the allegations against Peter Bone when these complaints were made.

“Boris Johnson must also be held accountable for his decision to appoint Peter Bone as deputy leader of the Commons despite the complaints made against him.”

She added in a letter to Case: “There are serious questions to be answered about what Boris Johnson, Liz Truss and other Conservative ministers knew about these allegations including when Peter Bone was appointed to a ministerial position.

“It is therefore crucial that any inquiry has full access to communications sent by Johnson and other ministers at the time, including emails, text messages and other electronic and written correspondence.”

Asked if an inquiry would be launched into Bone’s appointment as a minister, Rishi Sunak’s spokesperson said: “I am not aware of any plans for that, no.”

Bone has been one of most notorious backbenchers for many years, and often a thorn in the side to his own party leaders. An unashamed eurosceptic, he had described Ukip as a “good thing for British politics”. And also called the idea of introducing same-sex marriage “completely nuts”.