POLL: Who should be the next Conservative leader?

Mr Johnson summoned friendly Tory MPs to his Oxfordshire home on Sunday in likely preparation for a run at the party's leadership, as the Home Secretary was reportedly sounding out colleagues.

• MORE: Who should be the Labour leader?

Mrs May is thought to be the main contender to take on the former London mayor and a plot dubbed "ABB" (Anyone But Boris) has reportedly begun, organised by ministers and aides loyal to Mr Cameron.

Other challengers could include prominent Brexit campaigner and Secretary of State for Justice Michael Gove and Work and Pensions Secretary Stephen Crabb.

Meanwhile former defence secretary Liam Fox was the first potential contender to break cover, admitting he is "thinking about" standing to replace Mr Cameron.

Mr Johnson was pictured welcoming Remain campaigners Jake Berry, Amanda Milling and Ben Wallace, alongside Leave's Nigel Adams to his Oxfordshire home on Sunday.

Mrs May, touted as the "stop Boris" side's candidate, was also reported to be canvassing support among MPs ahead of the battle to replace the PM.

She has been silent since Friday's bombshell EU referendum result sent shockwaves through Britain's political system as the country questioned how Brexit could be delivered.

The pair, who are expected to announce their intentions this week, have been urged by Cabinet minister Justine Greening to form a "united leadership" to help bring together a country left divided after the poll.

The International Development Secretary said if Mr Johnson and Mrs May were unable to agree, another pair of MPs from either side of the referendum divide could step forward to "bring Britain back together".

Mr Cameron announced his intention to leave Number 10 in the wake of the referendum defeat and said he would like his successor to be in place by the time of the Tory party conference in October.

Mr Fox suggested that the timetable set by the PM should be extended to January 1 with candidates making their pitches at the October conference before MPs decide the two-person shortlist.

That would allow extra time for an outsider - such as Leave campaigner Mr Fox - to build momentum in their campaign for the leadership.

The Brexit-backer made his comments after former leader Iain Duncan Smith said the new Tory prime minister must come from the Leave camp.

Mr Duncan Smith's hardline stance would rule out Mrs May for the top job.

"It would be very, very difficult for the public who have voted for leaving the European Union to find that they then had a prime minister who actually was opposed to leaving the European Union," Mr Duncan Smith told BBC One's Andrew Marr Show.

The Conservatives should try to avoid a protracted leadership campaign because the party should "put our country first", according to Ms Greening.

She told Good Morning Britain: "I think the two clear front-runners are obviously going to be Boris Johnson and Theresa May.

"I would like to see them unite and bring our party together without the need for a leadership contest if we can possibly manage that."

Chancellor George Osborne gave no clues on whether he would be a candidate in the leadership election as he set out measures to calm the financial markets in an early-morning statement at the Treasury.

Mr Osborne said only that he would address the question of his future role in the Conservative Party in the coming days.

Asked whether he would serve in a Brexit-backing government, Mr Osborne said: "I take a simple view of life, which is 'It's my country, right or wrong' and I intend to fulfil my responsibilities to the country."