Whoever wins the leadership contest will succeed David Cameron as prime minister
Nominations for the Conservative leadership in Britain open later - formally firing the starting gun on the race to succeed David Cameron as British prime minister.
Senior Conservatives have confirmed they will be backing the favourite and leading Brexiteer Boris Johnson, while Stephen Crabb - the UK's Secretary of State for Work and Pensions - has made his pitch for the job, running on a "blue collar ticket" with business secretary Sajid Javid.
Meanwhile a survey for the ConservativeHome website has put Theresa May - who is expected to enter the race before Thursday's noon deadline - narrowly ahead of Mr Johnson among party members.
Cabinet ministers Liz Truss and John Whittingdale, from either side of the European Union referendum divide, are reported to have pledged support to Mr Johnson.
The Leave campaign frontman has also received the backing of Brexit-supporting justice minister Dominic Raab and farming minister George Eustice, with dozens more Conservative MPs expected to follow suit, The Sun reported.
Writing in the Daily Telegraph, Ms Truss, who backed the Remain side in the EU referendum, said: "Britain has achieved amazing things in the past and, with the right leadership, will do so again in the future.
"This leadership must now come from someone who believes in, and campaigned for, leaving the EU. The British people must be able to trust the new prime minister to deliver the instruction they have given. I do not think anyone who campaigned for us to stay in the EU would be able to secure that trust in the same way.
"That is just one reason why I am backing Boris Johnson to be the next leader of the Conservative Party and our next prime minister."
She also praised Michael Gove, the Brexit campaign chief, adding: "With him on Boris' team, I am convinced we can get a good deal for Britain."
Education Secretary Nicky Morgan, who is considering a leadership bid, said the Conservatives had to make the "positive case" for immigration rather than be "pushed around" by UKIP leader Nigel Farage.
Work and Pensions Secretary Mr Crabb has lined up Attorney General Jeremy Wright to act as the manager for his campaign.
Mr Crabb comes from the sort of ordinary background that appeals to many voters - and Business Secretary Mr Javid is the son of a Pakistani bus driver who became an investment banker.
Both men backed the Remain side although Mr Javid faced claims he was privately in favour of leaving the EU.
Setting out his stall in the Daily Telegraph, Mr Crabb pledged to unite the Conservatives on "a path of optimism and pragmatism".
He said he would take Britain out of the EU - with freedom of movement a "red line" as he seeks to craft a new immigration policy.
ConservativeHome said its survey of 1,315 members put Home Secretary Mrs May on 29% and Mr Johnson on 28%, with Andrea Leadsom and Liam Fox - neither of whom has declared their intentions - both on 13%.
Health Secretary Jeremy Hunt is also tipped to stand, while Chancellor George Osborne has ruled himself out of the race.
The new Conservative leader - who will move straight into Number 10 Downing Street - is expected to be in place by September 9th.