Britain's ruling Conservative Party begins selecting a new leader to replace David Cameron as prime minister on Tuesday with interior minister Theresa May and junior minister Andrea Leadsom the leading candidates to get the top job.

Cameron announced he would resign in the political maelstrom that followed when Britons voted on June 23 to leave the European Union despite his exhortations to remain, with his successor due in Downing Street by early September.

The leadership battle inside the Conservative Party has added to uncertainty at a time when Britain is facing the biggest political and economic upheaval since World War Two.

Five candidates have put their names forward and on Tuesday the 331 Conservative lawmakers in parliament will hold their first vote on who should be the next leader. Voting starts at 1000 GMT with the result announced about eight hours later and the candidate with the fewest votes will be eliminated.

The next round of voting will then take place on Thursday and the process will continue until just two candidates remain. The leader will then be elected by about 150,000 Conservative Party members across the country.

Theresa May, a Conservative stalwart who has run the security and law-and-order portfolio in Cameron's cabinet for six years, is the favourite with the bookmakers and to succeed and has the greatest backing among lawmakers.

However, May supported Britain staying in the EU and many Conservatives have argued the next prime minister who will need to negotiate Britain's exit from the bloc after 43 years membership needed to be someone who had supported leaving.

A poll for the ConservativeHome website put support among members for May on 37 percent, with 38 percent backing Leadsom, who was also prominent figure in the Brexit campaign.

Leadsom, 53, who had a 25 year career in financial services before turning to politics but has never served in cabinet, also received a boost on Monday when former London Mayor and leading Brexit campaigner Boris Johnson gave her his backing.

"ZAP"

"Andrea Leadsom offers the zap, the drive, and the determination essential for the next leader of this country," Johnson said in a statement.

"She has specialised in the EU question ... and will be therefore well-placed to help forge a great post-Brexit future for Britain and Europe," he said.

Johnson had been considered one of the favourites to replace Cameron but ruled out a bid last week after Justice Secretary Michael Gove, who had been widely expected to back him for the top job, unexpectedly announced his own candidacy.

Gove said he had changed his mind about his Brexit campaign colleague, as he did not think Johnson could provide leadership.

The other leadership contenders are work and pensions minister Stephen Crabb and former defence minister Liam Fox who is expected the candidate eliminated on Tuesday.

Both Britain's major parties have been hit by division and recriminations since the EU referendum, with opposition Labour Party leader Jeremy Corbyn also seeming set to face a challenge to his position.

He has refused to resign despite mass defections from parliamentarians in his policy team and a no confidence motion that was passed by an overwhelming majority of the party's lawmakers.

The party's deputy leader Tom Watson is due to meet trade union representatives, Labour's financial backers, in a "last throw of the dice" to try and reach a deal over Corbyn's leadership.