Rishi Sunak conceded that he “regrets” bringing Sir Gavin Williamson back into the Cabinet as he faces pressure in the wake of his ally’s resignation over bullying allegations.

The Prime Minister insisted on Wednesday he was unaware of “any of the specific concerns” relating to Sir Gavin’s tenure as defence secretary and chief whip.

But his limited denial effectively conceded he was aware of a complaint by former chief whip Wendy Morton, who received expletive-laden texts from Sir Gavin while he was a backbench MP.

Sir Gavin, who had already been twice sacked from the Cabinet in disgrace, bowed to pressure to resign as minister without portfolio on Tuesday after the allegations piled up.

A civil servant, then in the Ministry of Defence, alleged Sir Gavin told him to “slit your throat” before a Tory colleague alleged “unethical and immoral” behaviour while chief whip.

Labour leader Sir Keir Starmer used Prime Minister’s Questions to pressure Mr Sunak over how the allegedly bullied official felt at hearing him welcome the resignation “with great sadness”.

Mr Sunak responded: “Unequivocally the behaviour complained of was unacceptable and it’s absolutely right that the right honourable gentleman has resigned.

“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.”

Mr Sunak’s denial was limited to the allegations not centring on Ms Morton’s complaint, which relates to Sir Gavin’s time as a backbench MP.

The Prime Minister added: “I obviously regret appointing someone who has had to resign in the circumstances.

“But 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 Keir argued that Sir Gavin’s reputation as a “pathetic bully” was “precisely why the Prime Minister gave him a job”.

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

Mr Sunak had been warned when he took over the Tory party about Ms Morton’s complaint, which it later emerged centred on angry messages Sir Gavin sent as he complained about the lack of an invitation to the Queen’s funeral.

The Prime Minister’s press secretary said he was “made aware of a disagreement” between the pair but was “not aware of the detail of that disagreement” and “he was not aware of the content of the messages”.

She said Mr Sunak had “no awareness” at the time of the subsequent allegations that would emerge, first when the Guardian reported that the civil servant alleged Sir Gavin told him to “slit your throat” and on a separate occasion said, “jump out of the window”.

Former deputy chief whip Anne Milton alleged Sir Gavin used “unethical and immoral” methods while he was in charge of party discipline as chief whip.

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.

Ahead of PMQs, Education Secretary Gillian Keegan insisted Mr Sunak knew only about a “disagreement” between Sir Gavin and Ms Morton when handing him a role in the Cabinet Office.

“He didn’t know about any specific allegations, he hadn’t seen any text messages or anything like that,” she told LBC radio.

She added to BBC Breakfast: “I’ve worked with him for many years. He has integrity, he has fabulous judgment.”

Downing Street said Sir Gavin, who has previously been sacked as defence secretary and education secretary, offered his resignation to the Prime Minister in a face-to-face meeting on Tuesday night.

Sir Gavin issued a statement saying the allegations against him were “becoming a distraction for the good work this Government is doing for the British people” and was stepping back to “clear my name”.

In his response, Mr Sunak said he was accepting the resignation “with great sadness” and told Sir Gavin: “I would like to thank you for your personal support and loyalty.”

Labour’s deputy leader Angela Rayner suggested Sir Gavin should quit as an MP if he is found to have bullied colleagues or officials.

“There’s no place for bullies in Parliament,” she told BBC’s Newsnight.

She said Mr Sunak appointed Sir Gavin “with full knowledge of serious allegations about his conduct and repeatedly expressed confidence in him”.

Liberal Democrat deputy leader Daisy Cooper said: “Rishi Sunak has serious questions to answer about why he appointed Gavin Williamson, then stood by him instead of sacking him.”

With concerns over the internal Tory investigation into Sir Gavin being a “whitewash”, Ms Morton referred the case to Parliament’s bullying watchdog.

In a series of texts peppered with swear words, Sir Gavin accused her of seeking to “punish” MPs out of favour with then-prime minister Liz Truss by excluding them from the Queen’s funeral, warning: “There is a price for everything.”

Another complaint to Parliament’s Independent Complaints and Grievance Scheme (ICGS) has reportedly been made by the former senior official who worked with Sir Gavin at the MoD.

On Tuesday night, former deputy chief whip Ms Milton alleged Sir Gavin used intimidatory and threatening tactics while he was chief whip in 2016-17.

She accused him of seeking to use an MP’s financial situation as leverage against them and sending an expletive-laden email about a female civil servant.

Ms Milton described his behaviour as “unethical and immoral” and told Channel 4: “I think he feels that he’s Francis Urquhart from House Of Cards.”

Sir Gavin is a divisive figure at Westminster, where he is viewed with suspicion by many Tory MPs because of his reputation as an inveterate plotter.

He was sacked first by Theresa May as defence secretary in 2019 for leaking details of a National Security Council meeting, and then by Boris Johnson as education secretary over the Covid-19 A-levels debacle.