Rishi Sunak is under growing pressure to explain what he knew about the bullying allegations surrounding Dominic Raab before appointing him as Deputy Prime Minister.

Downing Street would only rule out the Prime Minister being aware of “formal complaints” as reports suggested he had been warned about his ally’s behaviour.

Mr Sunak was resisting demands from a civil servants’ union to suspend Mr Raab while under investigation, with dozens of officials believed to be behind eight formal complaints.

Labour leader Sir Keir Starmer accused the PM of being “too weak” to act and asked whether he was “the only person completely unaware” of the allegations.

How would he (Rishi Sunak) feel if one of his friends or relatives was being forced to work for a bully simply because the man at the top was too weak to do anything about it?

At a combative Prime Minister’s Questions, Mr Sunak insisted he had appointed lawyer Adam Tolley KC to investigate when he learned of “formal complaints”.

But reports have suggested that Mr Sunak was warned about Mr Raab’s conduct before appointing him as his deputy and Justice Secretary.

No 10 did not deny the Prime Minister had been made aware of “informal complaints”, pointedly sticking to Mr Sunak’s own wording.

His press secretary said: “The PM was not aware of any formal complaints at the time of appointing Dominic Raab.”

Pressed further, she said: “I don’t know what your definition of informal complaints is. The PET (propriety and ethics team in the Cabinet Office) processes are very clear.

“The appointments and usual processes were followed and we were not aware of any formal complaints.”

Three permanent secretaries who led officials working under Mr Raab at the Foreign Office, Ministry of Justice and Brexit department are thought to have spoken to the inquiry.

Sir Keir accused the Conservatives of having an “addiction to sleaze and scandal” as Mr Sunak’s party also grapples with allegations surrounding Boris Johnson and Nadhim Zahawi.

The Labour leader claimed Mr Sunak ignored reports about the tax dispute involving the now-sacked Tory chairman and had done similar about Mr Raab’s conduct.

Sir Keir highlighted reports suggesting “some of the complainants were physically sick, one says they were left suicidal”.

“How would he feel if one of his friends or relatives was being forced to work for a bully simply because the man at the top was too weak to do anything about it?” Sir Keir asked.

Mr Sunak said: “When I was made aware of formal complaints I appointed a leading independent KC to conduct an investigation, because I take action when these things happen.”

Sat next to the Prime Minister in the Commons, Mr Raab shook his head and muttered as the Labour leader questioned his appointment.

Mr Raab has always denied bullying but there are questions over why Mr Sunak appointed him despite allegations being made in public over many years.

The Prime Minister’s press secretary said: “Obviously he does occasionally look at news websites but it’s not something he does every morning with every newspaper, no.”

Dave Penman, leader of the FDA union, which represents senior officials, said Mr Raab should be suspended to protect other members of staff while the investigation is conducted.

“If that was any other employee, if that was a permanent secretary in the civil service, they would in all likelihood be suspended from their job while the investigation took place,” he told BBC Radio 4’s Today programme.

“That’s not to pre-judge the investigation, that’s to say if there are serious allegations of bullying and extensive allegations like this, that one of the considerations is how do you protect employees from that sort of behaviour? While it’s being determined you would normally suspend someone, given the seriousness and extent of those accusations.”

Lord Simon McDonald, the former top civil servant at the Foreign Office, Antonia Romeo, the current permanent secretary at the Ministry of Justice, and Philip Rycroft, who ran the Brexit department while Mr Raab was there, are thought to have given evidence as witnesses to the inquiry.

Mr Rycroft said “it would be a bit of a surprise if the investigation hadn’t reached out to those who were working with Dominic Raab through those years”.

He also told Times Radio on Wednesday: “I’m pleased the investigation is happening.

“Clearly for everybody involved in this, getting this sorted out is long overdue, but we’ll have to wait and see what the investigator comes up with, and, ultimately, the decision on the base of that evidence is one for the Prime Minister.”

Senior Conservative MP Sir Bernard Jenkin backed Mr Sunak for not suspending Mr Raab while under investigation, saying he was “entitled to due process whatever the hullabaloo”.

Sir Bernard said Mr Raab was a “demanding person to work with” but that officials should be prepared to work in very challenging situations.

“To an extent, if you’re at the top of the Civil Service or working closely with ministers, if you can’t stand the heat, get out of the kitchen – to an extent,” he told BBC Radio 4’s World At One.

His comments came a day after senior Tory Jacob Rees-Mogg warned people against being “too snowflakey” about bullying allegations as he defended Mr Raab.

The Liberal Democrats called on the Prime Minister to publish the advice given to him by the Cabinet Office when he appointed Mr Raab in October.

Deputy party leader Daisy Cooper said: “The public deserves to know the truth about what he knew and when, including the full disclosure of any advice given to him by the Cabinet Office.”