The former chancellor resigned from his ministerial post on Tuesday evening, just moments after Sajid Javid quit as health secretary.

It came as Mr Johnson faced further questions about his handling of the row over disgraced MP Chris Pincher, which ultimately led to him agreeing to step aside on Thursday.

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In his resignation letter to Mr Johnson, Mr Sunak said "the public rightly expect government to be conducted properly, competently and seriously", adding: "I believe these standards are worth fighting for and that is why I am resigning."

Announcing his bid, Mr Sunak said the country faces "huge challenges".

"Let's restore trust, rebuild the economy and reunite the country," he posted on social media.

In a video uploaded alongside his tweet pitching himself as the next Conservative leader, the former chancellor said: "Family is everything to me. And my family gave me opportunities they could only dream of.

"But it was Britain, our country, that gave them and millions like them the chance of a better future.

"I want to lead this country in the right direction."

The former chancellor also stressed his experience during the COVID pandemic as an example of why he is fit for the job.

"I ran the toughest department in government during the toughest times when we faced the nightmare of COVID," he continued.

"My values are non-negotiable: patriotism, fairness, hard work.

"We've had enough of division.

"Politics at its best is a unifying endeavour and I have spent my career bringing people together. Because that is the only way to succeed."

'We've had enough of division'

Mr Sunak's video address added that the UK cannot tell itself "comforting fairy tales".

"Do we confront this moment with honesty, seriousness, and determination, or do we tell ourselves comforting fairy tales that might make us feel better in the moment but will leave our children worse off tomorrow?," it continued.

"Someone has to grip this moment and make the right decisions. That's why I'm standing to be the next leader of the Conservative Party and your prime minister."

Mr Sunak said he would set out his vision in the "coming days and weeks".

Numerous Conservative MPs were quick to endorse Mr Sunak as their preferred leadership candidate following his bid.

Conservative MP Mark Harper said the former chancellor has an "exciting vision for the future of the country".

He posted on social media: "I'm backing Rishi Sunak to restore trust, rebuild the economy, reunite the country, win the next election.

"I'm #ReadyforRishi as he represents a return to traditional Conservative values and has an exciting vision for the future of our country."

Sunak 'can unite the party'

Fellow Conservative MP Angela Richardson, who quit her ministerial role as Michael Gove's parliamentary private secretary back in January citing "deep disappointment" with Mr Johnson's handling of the partygate scandal, also backed Mr Sunak for leader.

Former chief whip Mark Spencer was another to give his support to Mr Sunak.

"Rishi is the leader that can unite the party, bring the whole country together and win us that fifth general election victory.

"In serious times we need a person with a proven track record. Rishi gets my full support."

Red wall Tory MP Jacob Young also endorsed the former chancellor saying he has backed Teeside "time and time again".

"Rishi has the energy and vision to beat Labour in places like Redcar and I trust him to deliver for us with jobs, opportunities and a brighter future," he posted on social media.

Another Conservative MP from the 2019 intake, Laura Trott, posted on Instagram: "It is time for a leader with integrity. A leader we can trust to unite our party and bring the country together again."

"I am backing Rishi for his proven leadership through hard times and the vision to take the country forwards."

Claire Coutinho added: "Rishi is one of the most intelligent, decent and kind people I've ever worked with. He can restore trust in politics and get the economy back on its feet. That's why I think he is the right choice to lead our country"

Johnson's resignation starts firing gun on leadership race

Ben Wallace is currently the bookies' favourite to succeed Mr Johnson as Tory leader and prime minister, having risen up the party's popularity rankings in recent months.

The latest YouGov poll of Conservative Party members shows the defence secretary as the clear favourite for next leader.

Read more: Who are the frontrunners to replace Boris Johnson?

Mr Wallace beats all the main contenders including Mr Sunak, Penny Mordaunt and Foreign Secretary Liz Truss.

However, the defence secretary has yet to throw his hat in the ring.

Sky News' political correspondent Joe Pike understands Mr Wallace is discussing the matter with his family before deciding how to proceed

Addressing the nation just after midday on Thursday, Mr Johnson offered his resignation but said he intended to remain in office until his successor is elected, a process which could take months.

This prompted a backlash from senior Conservative figures, including former party leader Sir John Major, who in a letter to 1922 Committee chairman Sir Graham Brady said that it was "unwise and may be unsustainable" for Mr Johnson to stay in place until the autumn.

Labour leader Sir Keir Starmer has also threatened to call a vote of no confidence in the Commons, with the support of other opposition parties, if Tory MPs cannot oust him straight away.

Tugendhat the first to make bid

Mr Johnson's resignation did however fire the starting gun on the Conservative leadership election.

On Thursday evening, Tom Tugendhat has become the first MP to say he would run to replace Mr Johnson in a Tory leadership contest.

Read more: Ex-PM Theresa May 'angry' at partygate revelations engulfing successor Johnson

The ex-soldier and Foreign Affairs Select Committee chairman told Times Radio it would be a "huge privilege".

Transport Secretary Grant Shapps is believed to be considering a Conservative Party leadership bid.

But Sky News understands both Deputy Prime Minister Dominic Raab and former levelling up secretary Michael Gove have ruled themselves out of the contest.

Former health secretary Mr Javid, who dramatically resigned on Tuesday evening, is also understood to be deciding whether to enter the contest - according to the PA news agency.