"Enough is enough," said Mr Javid, as he declared he was no longer willing to "risk losing my integrity" by serving in Mr Johnson's government.

"I do fear the reset button can only work so many times," he told a packed House of Commons on Wednesday after quitting as health secretary the evening before.

"There's only so many times you can turn that machine on and off before you realise something is fundamentally wrong.

"Last month, I gave the benefit of doubt one last time but I have concluded that the problem starts at the top and I believe that it is not going to change."

Politics Hub: More resignations as pressure mounts on PM - live updates

As the PM looked uncomfortable on the frontbench, Mr Javid told MPs: "Treading the tightrope between loyalty and integrity has become impossible in recent months.

"I will never risk losing my integrity."

Mr Javid said he believes "a team is as good as its team captain" and "loyalty must go both ways" as he said it "is not fair" ministers had to appear on TV every morning defending lines that "don't stand up, don't hold up".

The now backbencher said he was done with giving the PM the benefit of the doubt and revealed he had been assured over partygate by the PM's top team "there had been no parties in Downing Street and no rules were broken".

He said he continued to give the benefit of the doubt following the damning Sue Gray report into partygate but after it turned out this week that Mr Johnson had known about 2019 allegations against MP Chris Pincher before he appointed him as deputy chief whip, Mr Javid said "at some point, we have to conclude that enough is enough".

Read more: Sajid Javid and Rishi Sunak's resignation letters in full

Mr Javid made a dig at his former cabinet colleagues who have not resigned.

"They will have their own reasons. But it is a choice, I know just how difficult that choice is," he said.

"But let's be clear, not doing something is an active decision."

The former health secretary kicked off a wave of resignations on Tuesday evening, sending a letter to Mr Johnson saying he could "no longer, in good conscience, continue serving in this government".

"It is clear to me that this situation will not change under your leadership - and you have therefore lost my confidence too," he wrote.

Cabinet member under three PMs

Mr Javid, who became an MP in 2010, was one of the most experienced members of the PM's cabinet, having served on David Cameron, Theresa May and Mr Johnson's front benches.

Under Mr Cameron, he was culture then business secretary, while Mrs May made him housing then home secretary.

The son of Pakistani immigrants, he was the first person of colour to hold one of the "great offices of state" when Mrs May appointed him home secretary in 2018 following the Windrush scandal.

He ran for the Tory Party leadership in 2016 and again in 2019, when he came fourth and ultimately lost out to Mr Johnson.

Seen as a safe pair of hands, he was handed the most senior job beneath the PM - chancellor.

But in February 2020 Mr Javid quit after just 204 days over a falling out with Mr Johnson's former right-hand man Dominic Cummings, who asked him to sack all his political advisers to remain in the role.

He was the first British chancellor in 50 years not to deliver a budget.

With Mr Cummings gone and Mr Hancock resigning over claims he broke social distancing rules with his aide, Mr Javid was brought back into government as health secretary last year.