THERESA May has vowed to fight an effort to oust her as Conservative leader and Prime Minister "with everything I've got".

In a dramatic early morning statement outside the door to 10 Downing Street, May warned a change of Prime Minister would put the UK's future at risk and could delay or halt Brexit.

She insisted she would stay on to "finish the job" she has set herself as Prime Minister.

May's statement came about an hour after it was confirmed the chairman of the Conservative backbench 1922 Committee, Sir Graham Brady, had received the 48 letters of no confidence from Tory MPs required to trigger a ballot on the leadership.

The vote will take place between 6pm and 8pm on Wednesday, with the result announced soon afterwards.

May needs to secure the votes of 158 MPs - half the parliamentary party plus one - to remain as Conservative leader, though a vote of 100 or more against her will raise questions about whether she can continue.

If she wins, another challenge cannot be mounted against her position as Tory leader for a year.

Immediate statements of loyalty for the Prime Minister were issued by Cabinet ministers, including several who have been named as potential contenders to be her successor.

Jeremy Hunt, Sajid Javid, Michael Gove, Philip Hammond, Matt Hancock, Penny Mordaunt, Liam Fox, Steve Barclay, David Lidington, Amber Rudd, Chris Grayling, David Gauke and James Brokenshire were among those making statements of support.

Standing before the famous door to Number 10, a defiant May said: "Sir Graham Brady has confirmed that he has received 48 letters from Conservative MPs so there will now be a vote of confidence in my leadership of the Conservative Party.

"I will contest that vote with everything I've got."

May said securing a Brexit deal which will deliver on the result of the 2016 referendum is "now within our grasp", and added she is "making progress" in securing reassurances from EU leaders on MPs' concerns about the proposed backstop for the Irish border.

But she warned: "A change of leadership in the Conservative Party now would put our country's future at risk and create uncertainty when we can least afford it."

A new leader could not be in place by the January 21 deadline by which the PM must confirm an agreement in principle had been reached with Brussels on a Brexit deal, she said.

"A leadership election risks handing control of the Brexit negotiation to opposition MPs in Parliament," she said.

"The new leader wouldn't have time to renegotiate a withdrawal agreement and get the legislation through Parliament by March 29, so one of their first acts would have to be extending or rescinding Article 50 - delaying or even stopping Brexit when people want us to get on with it."

A leadership election "would not change the fundamentals of the negotiation or the parliamentary arithmetic", she said.

"Weeks spent tearing ourselves apart would only create more division, just when we should be standing together to serve our country. None of that would be in the national interest.

"The only people whose interests would be served are Jeremy Corbyn and John McDonnell."

May said she wants to focus on voters' priorities like the economy, public services and housing, as well as delivering on the referendum vote.

She said she has devoted herself "unsparingly" to delivering her agenda of "building a country that works for everyone" since becoming PM, adding: "I stand ready to finish the job."

May said she had cancelled a planned trip to Dublin on Wednesday evening for talks with Irish Taoiseach Leo Varadkar in order to "make the case for my leadership" with MPs.

She is expected to speak with individual Tory MPs during the day before addressing the 1922 Committee at 5pm, immediately before the crucial vote.

Labour Party chairman Ian Lavery said: "With only weeks left before Britain leaves the EU, Theresa May's weakness and failure has completely immobilised the Government at this critical time for the country.

"The Prime Minister's half-baked Brexit deal does not have the backing of her Cabinet, her party, Parliament or the country.

"The Conservative Party's internal divisions are putting people's jobs and living standards at risk."