With turmoil surging through both of Britain’s main parties, defiant opposition Labour chief Jeremy Corbyn meanwhile faced a looming leadership challenge after last Thursday’s referendum.
Johnson, who led the “Leave” campaign to victory, tops the opinion polls along with interior minister Theresa May for the Conservative leadership, and therefore the keys to 10 Downing Street.
Work and Pensions Minister Stephen Crabb, a virtual unknown to the British public, has announced his intention to run, as has Nicky Morgan, who heads education policy.
As Cameron headed back from his last EU summit in Brussels, the Tory party prepared to open nominations for his successor at 5.00pm (1600 GMT). They close at noon Thursday, with the new leader expected to be announced on September 9.
Bookmakers make Johnson slight favourite over May, and The Sun newspaper reported Wednesday that he had already secured the backing of 100 of the Tory MPs who will vote next week to whittle down the field to two nominees.
The new leader will chosen by a postal ballot of party members, who currently number around 150,000.
The ConservativeHome blog surveyed more than 1,300 members and found the slimmest of leads for May, mirroring the results of a YouGov poll published Tuesday.
Pro-EU finance minister George Osborne, long seen as a possible Cameron successor, has ruled himself out.
Critics have questioned whether the “Leave” camp — and Johnson in particular — has any idea how to manage the unprecedented situation left by last week’s vote.
“He has still to offer anything like a concrete plan on how he would negotiate the post-Brexit future,” wrote former BBC political editor Nick Robinson.
The political turmoil was not confined to the ruling party, with opposition Labour leader Corbyn arming himself for battle against a huge majority of his own MPs.
Corbyn was defeated by 172 to 40 in a non-binding no-confidence vote held by Labour lawmakers late Tuesday, and speculation was mounting that a candidate would could forward and challenge him.
“It looks as though we will have a leadership election now,” Corbyn loyalist and shadow finance minister John McDonnell told reporters Wednesday.
Media reports suggested that his former business spokeswoman Angela Eagle could be preparing to launch a bid.
Corbyn is due later Wednesday to rally trade unions and the Labour members who helped win last year’s leadership vote by a landslide margin, and a fierce battle is looming.
“I was democratically elected leader of our party for a new kind of politics by 60 percent of Labour members and supporters, and I will not betray them by resigning,” the 67-year-old self-declared pacifist said in a statement.
The Labour turmoil comes amid speculation that Cameron’s successor as Conservative leader and prime minister may be tempted to call an early election later this year.
Labour, which has lurched to the left under Corbyn’s leadership, is viewed by some as unelectable in a general election while he remains in charge.
Former foreign minister Jack Straw said his party faced an “apocalypse” if an election was held with Corbyn still in office.
“It’s probably the worst crisis since Labour’s formation,” he told Sky News.
The leadership ructions of both parties dominated the front pages of Wednesday’s newspapers, with the Metro calling on Corbyn to “Please Jez go!” and The Sun declaring it “Boris day”.
In a sign of the depth of the crisis, the Scottish National Party, which has 54 seats in the House of Commons to Labour’s 229, announced it would make a bid to take over as the official opposition party in parliament.
“The official opposition must be ‘prepared to assume office’. Labour can’t anymore,” wrote SNP lawmaker Peter Wishart on Twitter, citing parliamentary practice.
He added that there was “more support” for the SNP’s government-in-waiting than the one cobbled together by Corbyn, after more than half of his top team resigned in protest at his leadership since the weekend.