London, June 29 The race to choose a successor to Prime Minister David Cameron got under way today after last week’s stunning Brexit vote, with former London mayor and top “Leave” campaigner Boris Johnson tipped as the narrow frontrunner. 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. But the first to throw his hat into the ring was work and pensions minister Stephen Crabb, a virtual unknown to the British public. The 43-year-old called on the party to “get past this Boris/stop Boris dichotomy”, in reference to the divisive Johnson. The party will officially open nominations at 5.00pm (1600 GMT). Johnson and May are expected to announce their bids tomorrow, before nominations close at noon. The new leader, who will be chosen by a postal ballot of party members currently numbering around 150,000, is expected to be announced on September 9. Bookmakers make Johnson slight favourite over May, and The Sun newspaper reported today 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 ConservativeHome blog surveyed more than 1,300 members and found the slimmest of leads for May, mirroring the results of a YouGov poll published yesterday. 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 chaos 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 come 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 today. Media reports suggested that his former business spokeswoman Angela Eagle could be preparing to launch a bid.