Gavin Williamson branded ‘a pathetic bully’ as his cabinet appointment is questioned

But Mr Sunak also told MPs he “did not know about any of the specific concerns” about his ally when he made the appointment, after he faced questions about his judgement.

Sir Gavin resigned as minister without portfolio on Tuesday after he was accused of sending abusive messages to a fellow Tory MP last month and of bullying a senior civil servant when he was Defence Secretary.

The MP, who is from Scarborough, said the allegations were “becoming a distraction for the good work this Government is doing” and he was stepping back to clear his name.

Labour leader Sir Keir Starmer described him as “a pathetic bully” and said he “would never get away with it if people like the Prime Minister did not hand him power”.

Speaking during Prime Minister's Questions, he added: “Everyone in the country knows someone in the country like the member for South Staffordshire, a sad middle manager getting off on intimidating those beneath him.

“But everyone in the country also knows someone like the Prime Minister, the boss who is so weak, so worried the bullies will turn on him that he hides behind them.

“What message does he think it sends when rather than take on the bullies, he lines up alongside and thanks them for their loyalty?”

Mr Sunak said Sir Gavin’s behaviour was “unacceptable” and he “obviously regrets” appointing him as a Cabinet Minister.

He said: “For the record I did not know about any of the specific concerns relating to his conduct as secretary of state or chief whip that date back some years.

“I believe that people in public life should treat others with consideration and respect and those are the principles that this Government will stand by.”

He added: “I think what the British people would like to know is that when situations like this arise that they will be dealt with properly.

“And that’s why it is absolutely right that he resigned and it’s why it is absolutely right that there is an investigation to look into these matters properly.”

Sir Gavin has been reported to MPs' bullying watchdog – the Independent Complaints and Grievance Scheme (ICGS) – after he allegedly sent expletive-laden messages to former chief whip Wendy Morton, complaining about being refused an invitation to the Queen’s funeral.

Another complaint was lodged after he was accused of bullying a civil servant at the Ministry of Defence and he also allegedly engaged in “unethical and immoral” behaviour while he was chief whip.

Earlier this week, Mr Sunak said he accepted the resignation “with great sadness” and told Sir Gavin: “I would like to thank you for your personal support and loyalty.”

Sir Gavin’s resignation led to fresh doubts among MPs about Mr Sunak’s political judgment, after he faced questions for reappointing Suella Braverman as Home Secretary after she was forced out for breaking the ministerial code.