• Previously-loyal supporters - including Priti Patel and Grant Shapps - urged him to step down on Wednesday.
• But Mr Johnson insists he has a "colossal mandate to keep going".
Image: BBC
Boris Johnson is fighting to stay on as prime minister as the wave of resignations from his government and party since Tuesday night exceeds 50.
Northern Ireland Secretary Brandon Lewis became the latest cabinet member to quit early on Thursday, quickly followed by six other ministers.
Previously-loyal supporters - including Priti Patel and Grant Shapps - urged him to step down on Wednesday.
But Mr Johnson insists he has a "colossal mandate to keep going".
Meanwhile, Attorney General Suella Braverman has said she will stand in any leadership challenge, although she did not resign.
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In his resignation letter on Thursday, Mr Lewis said: "I have given you, and those around you, the benefit of the doubt. I have gone out and defended this government both publicly and privately. We are, however, now past the point of no return."
Treasury minister Helen Whately, security minister Damian Hinds, science minister George Freeman, pensions minister Guy Opperman, technology minister Chris Philp, and courts' minister James Cartlidge also resigned in swift succession.
Taking aim at his critics, the prime minister sacked Levelling Up Secretary Michael Gove in a shock move on Wednesday, with a Downing Street source describing Mr Gove as a "snake" who "gleefully briefs the press that he has called for the leader to go".
Mr Gove, a former ally in the Brexit campaign but who derailed Mr Johnson's first bid for the Tory leadership, had urged the PM to resign earlier in the day.
On Wednesday, more than 40 ministers and aides quit - a record for a 24-hour period.
Even late into the night, the resignations continued, with Welsh Secretary Mr Hart standing down just before 23:00 BST.
Mr Hart said colleagues had done their utmost in private and in public "to help you turn the ship around, but it is with sadness that I feel we have passed the point where this is possible".
He had been among a group of cabinet members who attempted to persuade the prime minister to stand down, which also included Mr Johnson's former close allies Home Secretary Ms Patel, Transport Secretary Mr Shapps and Business Secretary Kwasi Kwarteng.
Later on Wednesday night, former loyalist Ms Braverman joined the calls for Mr Johnson to stand down, telling ITV's Peston that he had handled matters "appallingly" in recent days.
She said she would not resign as it was her duty to carry on in her current job - but said: "If there is a leadership contest, I will put my name into the ring."
Former Health Secretary Matt Hancock also withdrew his backing for the PM, saying he had "supported him through thick and thin" but he now needed to go.
Mr Hancock - who said he would not be running for the leadership - predicted Mr Johnson would not be leader for much longer, "whether that's tomorrow or next week".