David Cameron chaired his final cabinet meeting on Tuesday after six years as Britain's prime minister, as incoming premier Theresa May prepared to form a new government to deliver Brexit.

May led tributes to Cameron at the meeting, which was described by ministers as "emotional", and posed for photographers on the steps of the premier's 10 Downing Street residence afterwards.

Cameron's end has come sooner than expected after dramatic twists in the contest to replace him led to his swift exit from power less than three weeks after the nation's seismic vote to quit the European Union.

May is already under pressure to set out a timetable for Brexit from EU leaders who warn that a delay could prolong damaging economic uncertainty.

"That's what I think a lot of people expect and hope and call for," the European Commission's economy chief Pierre Moscovici said in Brussels.

On a visit to the headquarters of the governing centre-right Conservative Party she now leads, May issued an appeal for unity following a referendum campaign that divided the country.

"Now, more than ever, we need to work together, to deliver on Brexit, to build a country that works for everyone, and to truly unite our party and our country," she said.

She dismissed calls for an early general election -- the next one is not due until 2020 -- saying the Conservatives should use the next four years to build support.

The opposition Labour party is currently in no state to contest an election, as its leader Jeremy Corbyn battles a revolt by MPs.

In a major boost to his efforts to keep his job, the party's National Executive Committee ruled late Tuesday that he would automatically be included on a leadership ballot, setting up a contest with rival MP Angela Eagle.

May mulls cabinet picks

May, 59, will become Britain's second female prime minister after Conservative titan Margaret Thatcher.

Cameron announced he would step down after leading the failed campaign for Britain to remain in the EU in the June 23 referendum.

May, his interior minister for the past six years, was declared the new Conservative leader on Monday after junior energy minister Andrea Leadsom, her only remaining challenger for the post, withdrew from the contest.

One of May's first decisions will be when she plans to trigger Article 50 -- the formal procedure for withdrawal from the EU -- which would set a two-year deadline for completing exit negotiations.

While she supported Britain staying in the bloc, she maintained a low profile during the referendum campaign and insists she will honour the popular vote, stressing repeatedly: "Brexit means Brexit".

"The sooner the negotiations start, the better it will be for everybody," said French Foreign Minister Jean-Marc Ayrault.

Cameron will face MPs in parliament for a final time on Wednesday in the weekly prime minister's questions session, before meeting Queen Elizabeth II to tender his resignation.

The monarch will then invite May, the leader of the majority party in parliament, to form a fresh government.

As she fills the major roles in her government, May will have to keep Brexit-supporting heavyweights onside if she is to heal the splits in the party caused by the referendum.

On the markets, the pound rebounded against the dollar, shooting back above $1.30 on the political developments.

Labour opposition in turmoil

As removal men arrived in Downing Street to help Cameron and his family pack up, Corbyn won a boost for his own efforts to stay as Labour leader.

He has refused to quit despite losing a confidence vote by 172 votes to 40, pointing out that he was elected on a strong mandate from grassroots party members only last September.

Eagle launched a challenge on Monday, after months of criticism over Corbyn's performance crystallised over his perceived lacklustre efforts to keep Britain in the EU.

She and any other potential leadership candidates will need the support of 20 percent of lawmakers to stand -- but the NEC vote means that Corbyn will not, and can focus on rallying his grassroots supporters.

Speaking to cheering fans outside the meeting, Corbyn said he was "delighted" that he had been confirmed on the leadership ballot and would campaign across the country on issues of inequality, poverty and opportunity.

"That will strengthen our party in order to defeat this Tory government and bring in a government that cares for the people," he said.

Eagle had earlier urged Corbyn to "get control" of his supporters after a brick was hurled through her office window. He condemned the violence and called for calm.