THERESA May is to be the UK's second female prime minister after her only remaining rival, Andrea Leadsom, sensationally dropped out of the race to succeed David Cameron today.

Mrs May swiftly received backing from leading Brexit supporters Michael Gove and Boris Johnson, and the chairman of the Conservatives' backbench 1922 committee, Graham Brady, said there was no need for the leadership contest to be re-run.

And David Cameron said Mrs May will take the keys to No 10 by Wednesday evening.

The Prime Minister said he was "delighted" that the Home Secretary would succeed him in Downing Street and he will go to Buckingham Palace to offer his resignation to the Queen on Wednesday afternoon.

Mr Cameron will chair his last Cabinet meeting tomorrow morning and appear in the Commons for the final time as premier when he takes to the Despatch Box for Prime Minister's Questions at noon on Wednesday.

Mrs May had launched her national campaign with a speech in Birmingham just moments before Mrs Leadsom's withdrawal today.

"Theresa will do everything she can to equip our country for the challenges that lie ahead," said her campaign manager, Chris Grayling.

Mrs Leadsom announced that she was quitting the contest shortly after apologising to the Home Secretary over an interview in which she appeared to suggest that the fact she was a mother gave her the edge over the childless Mrs May as a future PM.

Speaking on the steps of her campaign HQ in Westminster, the Energy Minister wished Mrs May "the very greatest success" and promised her "my full support".

Mr Gove, who came third in the leadership ballot among Tory MPs, pledged his backing to Mrs May, saying: "Andrea Leadsom spoke with great dignity and courage today. I wish her every success in the future.

"We should now move as quickly as possible to ensure Theresa May can take over as leader. She has my full support as our next prime minister."

And Mr Johnson - who abandoned an expected tilt for the leader's job after realising he faced competition from both Mrs Leadsom and Mr Gove - said he had "no doubt" that Mrs May would make an excellent party leader and PM.

Mr Johnson said: "Theresa May will provide the authority and the leadership necessary to unite the Conservative Party and take the country forward in the coming weeks and months.

"Andrea's decision, which is both brave and principled, allows that process to begin immediately.

"I have no doubt Theresa will make an excellent party leader and Prime Minister and I'm encouraged that she's made it clear that Brexit means Brexit - that we will leave the EU.

"It is vital that we respect the will of the people and get on with exploiting new opportunities for this country."

The handover of power to a new prime minister had not been expected to take place until after the conclusion of the ballot of 150,000 Conservative members on September 9.

There were demands for Mrs May to call a snap general election, rather than waiting for the 2020 contest envisaged under Mr Cameron's fixed-term parliaments legislation.

Liberal Democrat leader Tim Farron said: "With Theresa May's coronation we need an early general election. The Tories now have no mandate. Britain deserves better than this."

And Green MP - and leadership candidate - Caroline Lucas said: "The case for an early general election (is) clearer than ever. Must not be a prime ministerial coronation."

Shadow cabinet minister Jon Ashworth said a "coronation" of Mrs May now looked likely, but added that she would have "nothing new" to offer working people, despite her claim in a speech this morning that she would create "a country that works not for the privileged few, but for every one of us".

Angela Eagle, who formally launched her own bid for the Labour leadership as the dramatic scenes were unfolding at Westminster, was asked why she thought she would be able to defeat Mrs May at an election. She replied: "Because she's a Tory."