In a huge blow to embattled British Prime Minister Boris Johnson, two of his senior Cabinet colleagues, including Chancellor Rishi Sunak, resigned on Tuesday, as they expressed their loss of confidence in his leadership amidst a spate of scandals.

The 42-year-old British Indian minister posted his resignation letter on Twitter soon after another senior Cabinet colleague, UK Health Secretary Sajid Javid, resigned.

The ministerial exits will come as a big blow to Johnson's leadership and follow a day of high political drama since a former civil servant spoke out about Downing Street's handling of allegations against recently suspended MP Chris Pincher.

"The public rightly expect government to be conducted properly, competently and seriously," Sunak tweeted.

"I recognise this may be last ministerial job, but I believe these standards are worth fighting for and that is why I am resigning," he said.

It came soon after Johnson said he "bitterly regrets" giving Pincher a government role as Deputy Chief Whip after being made aware of a misconduct complaint against him.

"In hindsight it was the wrong thing to do and I apologise to everyone who has been badly affected by it. I just want to make absolutely clear that there's no place in this government for anybody who is predatory or abuses their position of power," he said.

Sajid Javid, a British citizen of Pakistani origin, in his resignation letter said, "We (Conservative party) may not have always been popular, but we have been competent in acting in the national interest. Sadly, in the current circumstances, the public are concluding that we are now neither.

"The vote of confidence last month showed that a large number of our colleagues agree.

"I regret to say, however, that it is clear to me that this situation will not change under your leadership and you have therefore lost my confidence too," Javid, 52, said.

Earlier on Tuesday, Lord Simon McDonald, who was the permanent secretary in the UK Foreign Office between 2015 and 2020, wrote to the Parliament's standards commissioner saying Downing Street made "inaccurate claims" about Chris Pincher who resigned from the post of Conservative Deputy Chief Whip last week after admitting drunken misbehaviour.

In the wake of the resignation, the Prime Minister's office said Boris Johnson was not aware of any specific allegations against Pincher when he was appointed to the post.

"This morning I have written to the Parliamentary Commissioner for Standards because No. 10 [Downing Street] keep changing their story and are still not telling the truth," McDonald said as he tweeted his letter to the Parliamentary watchdog.

In the letter, he wrote: "The original No. 10 line is not true and the modification is still not accurate. Mr Johnson was briefed in person about the initiation and outcome of the investigation.

Sajid Javid File Picture

"There was a 'formal complaint'. Allegations were 'resolved' only in the sense that the investigation was completed; Mr Pincher was not exonerated. To characterise the allegations as 'unsubstantiated' is therefore wrong."

Deputy Prime Minister Dominic Raab was asked about these developments on air on Tuesday and defended his boss, saying it was not his understanding that Boris Johnson was directly briefed.

In relation to the 2019 allegation or complaint (against Pincher), whilst there was inappropriate behaviour, it didn't trip the wire into disciplinary action," he told the BBC.

Pincher, who was suspended as a Conservative Party MP for Tamworth last week over allegations he groped two men at a private members' club in London, says he is seeking professional medical support and has no intention of resigning as an MP.

The Opposition Labour Party said it was clear that Boris Johnson knew about the seriousness of the complaints against Pincher when he appointed him to an important post of Deputy Chief Whip, in charge of party discipline.

"He refused to act and then lied about what he knew. Boris Johnson is dragging British democracy through the muck. His appalling judgement has made Westminster a less safe place to work," said Labour Deputy Leader Angela Rayner.

For months Prime Minister Johnson has been facing a barrage of criticism and rebellion over his conduct and that of his government, including illegal, Covid-linked lockdown-breaking parties at his Downing Street offices for which he and others were fined by the police.

Last month, Johnson survived a confidence vote, but the final count of his lawmakers who rebelled against him was higher than his supporters expected: 41 per cent of his own Conservative parliamentary party refused to back him.

Meanwhile, another senior Indian-origin minister, Home Secretary Priti Patel, has said she has no plans to follow her other South Asian origin colleagues out of Cabinet.

At the same time, Justice Secretary Dominic Raab, Business Secretary Kwasi Kwarteng, Defence Secretary Ben Wallace, Foreign Secretary Liz Truss, and International Trade Secretary Anne-Marie Trevelyan have also expressed their support for Johnson.