Theresa May was facing demands for an immediate general election after securing her place as the UK's second female prime minister through the surprise withdrawal of her only rival in the battle to succeed David Cameron.

Energy Minister Andrea Leadsom offered Mrs May her "full support" after conceding that she had too little support among Tory MPs to deliver a "strong and stable government".

Andrea Leadsom MP speaks to the media as she announces her withdrawal from the Conservative leadership race at Cowley Street race on July 11, 2016 in London, England. Leadsom has pulled out of the Conservative leadership race to be Prime Minister of the United Kingdom, and will now offer her full support to Theresa May MP. (Photo by Carl Court/Getty Images)

And, crucially, the figureheads of the Brexit campaign in last month's referendum, Michael Gove and Boris Johnson, offered the Home Secretary their backing as new Conservative leader.

The chairman of the Conservative 1922 Committee, Graham Brady - who oversees Tory leadership contests - said he had to consult the party's board before formally confirming Mrs May as new leader. But he made clear that there was no need to re-start the election, and there was no doubt at Westminster that it was simply a matter of time before she becomes Prime Minister.

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Downing Street sources said discussions were under way about the timing of Mr Cameron's departure.

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 but is now expected to happen within days - and possibly before the end of Monday.

Labour election co-ordinator Jon Trickett said he was putting the whole party on general election footing, on the very day when its own leadership contest was kicked off by a formal challenge to Jeremy Corbyn from Angela Eagle.

And the Liberal Democrats and Greens demanded an early election following Mrs May's "coronation".