British Prime Minister Boris Johnson on Thursday said he was sad to announce his resignation as Conservative Party leader, triggering a leadership election for a new Tory leader who will go on to become the new PM.

Johnson, 58, said he will remain in charge at 10 Downing Street until the process of electing a new leader is completed - expected by the time of the Conservative Party conference scheduled for October.

"I want you to know how sad I am to be giving up the best job in the world," Johnson said.

He blamed the "herd instinct" of his party as he delivered his resignation speech on the doorstep of 10 Downing Street.

"It is clearly now the will of the parliamentary Conservative Party that there should be a new leader of that party and therefore a new Prime Minister,” he said.

"I have agreed with Sir Graham Brady, the chairman of our backbench MPs that the process of choosing that new leader should begin now and the timetable will be announced next week and I have today appointed a Cabinet to serve as I will until a new leader is in place," he said.

The outgoing leader reiterated he had been given an "incredible mandate" at the 2019 general election, which was "the reason I have fought so hard in the last few days to continue to deliver that mandate in person”.

"I felt it was my job, my duty, my obligation to you to continue to do what we promised in 2019," he said.

Referring to the high drama of the last few days, Johnson said he had tried to persuade his colleagues that it would be “eccentric” to change governments with such a “vast mandate” and when the Tories are only a “handful of points” behind in the polls and when the economic scene is so difficult domestically and internationally.

He added: "I regret not to have been successful in those arguments and of course it is painful not to be able to see through so many ideas and projects myself.

"As we have seen at Westminster, the herd instinct is powerful and when the herd moves, it moves and my friends in politics no one is remotely indispensable and our brilliant Darwinian system will produce another leader, equally committed to taking this country forward through tough times." The move came after days of high drama and a steady stream of resignations from his Cabinet since Tuesday and just minutes after his newly appointed Chancellor to replace Rishi Sunak wrote a public letter calling for him to “go now”.

Nadhim Zahawi, the Iraqi-origin minister seen as a frontrunner to replace Johnson in 10 Downing Street, wrote a damning letter which openly questioned his boss’ authority and demanded his exit. While not officially resigning from his new Cabinet post, the 55-year-old minister said the time was up for Boris Johnson.

"Prime minister, you know in your heart what the right thing to do is, and go now," he signs off.

His demand followed over 50 resignations since Johnson’s apology on Tuesday, after days of changing messages from Downing Street over the hiring and handling of the now-suspended Chris Pincher, who quit as Deputy Chief Whip in charge of Tory party discipline last week after admitting drunken misconduct. It comes in the wake of the partygate scandal of COVID law-breaking parties in Downing Street, which had resulted in a no-confidence vote last month which Johnson just about survived.

Leadership battle

His resignation will now trigger a fierce leadership battle within the Conservative Party. The 1922 Committee is responsible for setting the timetable for a Tory leadership contest. To take part in the race, a Tory MP has to be nominated by eight colleagues. If more than two MPs put themselves forward and secure enough nominations to run for leader, a series of secret ballots are held to whittle them down.

UK Attorney General Suella Braverman, who chose not to resign from Cabinet saying her legal role made it incumbent upon her to carry on, had withdrawn her support from Johnson and threw her hat in the ring to take over as the next Conservative Party leader and future Prime Minister.

"It pains me to say it, I have been an admirer and long-standing supporter of the Prime Minister but the facts are undeniable that he can’t command the confidence of a sufficient number of people to serve in his government,” the Goan-origin minister told the BBC.

The pro-Brexiteer, who described herself as a “steadfast” supporter of Johnson so far, said she was keen to step in to be able to fix the remaining issues with the Brexit deal and get things done.

"The writing’s on the wall, I can’t fight that any more. I have run the Attorney General’s office. I am putting myself forward because I believe that the 2019 manifesto is fit for purpose, presents a bold and inspiring vision for our country and I want to deliver on the promises contained in that manifesto. I want to embed the opportunities of Brexit and tidy up the outstanding issues… and cut taxes,” she said.

PM aspirants

Braverman, 42, is so far the only Tory MP to formally declare her leadership bid. Others expected to join the race include Zahawi and Foreign Secretary Liz Truss, with former Chancellor Rishi Sunak and former Health Secretary Sajid Javid among some of the others seen as likely contenders.

Indian-origin Home Secretary Priti Patel is also among the names touted along with UK Defence Secretary Ben Wallace. Among the more junior members, Tory MPs Penny Mordaunt, Tom Tugendhat and former Foreign Secretary Jeremy Hunt are the names doing the rounds.