Home Secretary Theresa May and former mayor of London Boris Johnson are set to enter the race to become the next Tory leader and prime minister.
They will declare their candidacies, setting out how much support they enjoy and how they propose to unify the party after the Brexit vote.
Ex-minister Liam Fox is also throwing his hat into the ring, joining Work and Pensions Secretary Stephen Crabb.
The outcome of the contest to succeed David Cameron is due on 9 September.
Mr Cameron, who has been Tory leader for 10 years and led the country since May 2010, is stepping down after losing the vote on the UK’s future membership of the EU.
He has said fresh leadership is required to take on the task of negotiating the UK’s separation from the EU.
The make-up of the field will become clear when nominations close on Thursday, although Mrs May, Mr Johnson and Dr Fox are expected to announce their intentions in a series of speeches in London.
Mrs May, who has been home secretary since 2010, told the Times newspaper that she was capable of “uniting Britain” and healing the divisions exposed by the Brexit result.
“Following last week’s referendum, our country needs strong, proven leadership to steer us through this period of economic and political uncertainty, and to negotiate the best possible terms as we leave the European Union,” she wrote.
The BBC understands Mrs May will announce a plan for a senior minister with a dedicated department to focus on the Brexit negotiations.
She has already won the support of cabinet colleague Justine Greening who told the Evening Standard that she had the “steeliness and professionalism” to secure the best outcome for the UK in Brexit negotiations and was also a “role model” for women coming into politics.
In the Times article, Mrs May calls for an ambitious programme of social reform to improve the life chances of those from disadvantaged backgrounds for whom job insecurity is the norm.
She also stresses her long experience in a top government role.
“Frankly, not everybody in Westminster understands what it’s like to live like this,” she writes. “And some need to be told that what the government does isn’t a game, it’s a serious business that has real consequences for people’s lives.
She also wrote that the role of the state needed to be thought of differently.
“Instead of thinking of it always as the problem, we should acknowledge that often it is only the state that can provide solutions to the problems we face.
“So yes, the state needs to be small, but it needs to be strong.”
In contrast to Mr Johnson, who was a leading supporter of the UK leaving the EU, Mrs May backed a Remain vote although she kept a low profile during the campaign and also indicated she would favour restrictions to freedom of movement rules if the UK stayed in.
Mr Johnson is expected to place Brexit at the heart of his “optimistic vision” for the country, saying the greater self-determination that leaving the EU will bring is an opportunity to “believe in ourselves and the values of our country”.
The ex-London mayor has won the backing of figures on other sides of the EU referendum argument, including Environment Secretary Liz Truss who said his two victories in London showed his ability to “reach out” to people beyond the Conservative base.
She has said the Brexit vote meant “leadership must now come from someone who believes in, and campaigned for, leaving the EU”.
Mr Johnson’s campaign is being run by Justice Secretary Michael Gove, who was also a central figure in the Leave campaign. But an e-mail from Mr Gove’s wife Sarah Vine to her husband, which emerged on Wednesday, suggested possible obstacles to a Johnson victory.
In it, she urged her husband to get “specific assurances” from Mr Johnson about what role he would have in a future government.
The e-mail, in which a member of the public is mistakenly believed to have been copied in, suggested party members, and press barons like Rupert Murdoch, would not have the “necessary reassurance” to back Mr Johnson without Mr Gove’s input.
The two-month contest is inevitably being seen through the prism of the Brexit vote and who is best placed to negotiate an exit from the EU which protects the UK’s economic interests while meeting the expectations of those who voted leave about controls on immigration.
Dr Fox, who finished third in the 2005 leadership election and was defence secretary between 2010 and 2012, said there could be “no backsliding” on referendum pledges on immigration, tweeting that “access to the single market does not require free movement”.
He later told LBC Radio that “experience and background” were important characteristics in a future leader and that the leadership campaign could not be totally dominated by Europe – and that issues such as health and defence must feature too.