The prime minister's official spokesman said the claims related to Mr Raab's previous tenure as justice secretary.

It means a total of eight complaints are now being investigated by senior lawyer Adam Tolley KC.

Mr Raab, who was reappointed by Rishi Sunak as justice secretary and deputy prime minister in October, has denied allegations of bullying.

Appearing in front of Parliament's Joint Human Rights Committee on Wednesday, Mr Raab said he had "behaved professionally throughout".

He told MPs he welcomed the investigation so he could deal with claims "transparently", rather than through "tittle tattle that's anonymously leaked to the media".

The MP for Esher and Walton previously served as justice secretary and deputy prime minister under Boris Johnson.

A close ally of Mr Sunak, Mr Raab was sacked from those roles when Liz Truss became prime minister in September.

The three complaints already under investigation related to his time as foreign secretary and Brexit secretary, as well as at the Ministry of Justice.

A spokesman for Labour leader Sir Keir Starmer said it was "pretty shocking" that no action had been taken against Mr Raab and called for him to be suspended.

He said it was "a consequence of having a weak prime minister" that Mr Raab continued to serve in government while complaints about his behaviour were investigated.

The Liberal Democrats also called for Mr Raab to step down as justice secretary while the complaints were investigated.

Deputy leader Daisy Cooper said: "The trickle of allegations about Dominic Raab has turned into a flood and his position is becoming increasingly untenable."

Asked how Mr Raab could remain in his cabinet role considering the allegations against him, the prime minister's official spokesman said: "We think it's right there is an independent process, that the investigator looks into these claims thoroughly before coming to a view."

He added that the investigation would be concluded "swiftly".

Mr Tolley, a commercial and employment law specialist, was appointed by the prime minister to investigate complaints against Mr Raab in November.

He was chosen to lead the investigation because the role of adviser on ministers' interests has been vacant since Lord Geidt's resignation in June.

The lawyer will report to Mr Sunak, who will make the final judgement on whether Mr Raab's conduct breached the ministerial code and should be sacked.