Tory leadership: Iain Duncan Smith backs Andrea Leadsom
Former Conservative leader Iain Duncan Smith has said he backs Andrea Leadsom to be the UK's next prime minister.
They campaigned together for the UK to leave the EU, and Mr Duncan Smith said Mrs Leadsom was the "right person to lead" in the forthcoming negotiations.
Meanwhile, Mrs Leadsom said the new PM must be a Leave supporter - which would rule out frontrunner Theresa May.
The other candidates to replace David Cameron are Stephen Crabb, Michael Gove and Liam Fox.
As of 18:00 BST on Friday, Mrs May had secured the backing of 96 Tory MPs, while Mr Crabb had 22, Mrs Leadsom had 21, Mr Gove had 18 and Mr Fox had 10.
More than 160 Conservative MPs had yet to declare whom they would support.
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Whoever is elected Conservative leader will succeed Mr Cameron as prime minister and be responsible for negotiating the UK's exit from the EU, following the 23 June referendum result.
The candidates will take part in a series of ballots of the party's 330 MPs, starting on Tuesday.
The two most popular will then go on to a vote of the wider party membership, with the result due on 9 September.
By BBC political correspondent Chris Mason
Prime Minister Leadsom? After the last week or so in politics, nothing sounds ridiculous.
Here's the theory behind those talking her up.
After what plenty of Conservative MPs see as Michael Gove's act of Olympian skullduggery in finishing off Boris Johnson, could pro-Brexit Tories back Mrs Leadsom instead?
And, if she made the final pair put forward for rank and file party members to pick from, could they back her over Home Secretary Theresa May, who wanted a Remain vote in the referendum?
My vow given in the last week is to give up making predictions.
The one fact is this. While the time frame is tight - the first round of voting in the Tory leadership race is Tuesday, the next round Thursday - things are "fluid".
That's a polite way of saying: Who on earth knows what will happen?
I'm not sure I've got the foggiest idea any more.
Mr Duncan Smith said he had spoken to all the candidates and had "huge confidence" in Mrs Leadsom, the energy minister.
He praised her strength, experience and "ability to achieve objectives even against considerable odds".
"I believe that Andrea's strong family background, business experience, compassion, commitment to social justice and dedication will make her a great prime minister for the UK," he added.
Who's backing who?
In an interview with the Daily Telegraph, Mrs Leadsom said the new PM should be someone who had supported Brexit.
Though she did not directly mention Mrs May - who supported Remain - she said it would be "very difficult" for someone who had backed staying in the EU to lead the Tory party.
She also said "key negotiating people" in the UK's EU exit talks would have to be Brexit supporters.
Launching his leadership bid with a speech on Friday, Mr Gove said his candidacy was driven by "conviction" about what was right for the UK, not personal ambition.
The justice secretary said that when he concluded Boris Johnson was not the "right person" to succeed Mr Cameron, his heart told him he should put himself forward.
Mr Gove, who said he had a "burning desire to transform our country", vowed to cut immigration and increase NHS spending if he became PM.
But he is facing calls to step down to ensure party unity, and business minister Anna Soubry said he had "behaved appallingly" in pledging support for Mr Johnson then withdrawing it at the last minute.
Asked whether he had been betrayed by Mr Gove, Mr Johnson replied that he had not been able to "get on with what I wanted to do" and it was now "up to somebody else".
Mrs May has been boosted by the support of the Daily Mail and cabinet colleagues including Defence Secretary Michael Fallon.
Mr Fox, meanwhile, said the focus should be on how the UK would negotiate its EU exit to get the best possible deal, saying anything else was a distraction.
Who's in the running?
Home Secretary Theresa May: The 59-year-old has replaced Boris Johnson as the bookies' favourite to win the contest. She's held the Home Office brief - often something of a poisoned chalice - since 2010, and is a former Tory party chairman. She says she can offer the "strong leadership" and unity the UK needs, and promised a "positive vision" for the country's future. She backed staying in the EU. Theresa May profile
Justice Secretary Michael Gove: The 48-year-old former newspaper columnist was a key figure in the party's modernisation that led to its return to power in 2010. He was a reforming, if controversial, education secretary between 2010 and 2014, and now holds the Ministry of Justice brief. He was a leading player in the Brexit campaign - which put a strain on his close friendship with David Cameron. He has pitched himself as the candidate that can provide "unity and change". Michael Gove profile
Work and Pensions Secretary Stephen Crabb: The 43-year-old was promoted to the cabinet in 2014 as Welsh secretary, and boosted his profile earlier this year when he took over as work and pensions secretary. A rising star of the Tory party he has promised to unite the party and country following the referendum result and provide stability. Raised on a council estate by a single mother, he has a back story to which many Tory MPs are attracted. Backed Remain. Stephen Crabb profile
Energy minister Andrea Leadsom: The 53-year-old former banker and fund manager was one of the stars of the Leave campaign. A former district councillor, she became MP for South Northamptonshire in 2010 and - after serving as a junior Treasury minister and as a member of the Treasury select committee - she was made a junior minister in the energy and climate change department in May last year. Andrea Leadsom profile
Former cabinet minister Liam Fox: It's second time around for the 54-year-old ex-defence secretary and GP, who came a close third in the 2005 leadership contest. His cabinet career was cut short in 2011 when he resigned following a lobbying row. A Brexit campaigner, and on the right of the party, he has said whoever becomes PM must accept "the instruction" of the British people and not "try to backslide" over EU membership. Liam Fox profile