Tory heavyweights Boris Johnson and Theresa May will set out their competing visions for Britain outside the EU as they formally enter the race to succeed David Cameron in Downing Street.
Mr Johnson, who led the Leave campaign in the referendum, will use his launch speech to present what was described as a "positive, optimistic vision" of Britain outside the EU, offering "a chance to believe in ourselves".
In her launch, Mrs May will promise to reunite both the Conservative Party and the country in the wake of the referendum campaign while securing the best deal possible for the UK in the negotiations with Brussels.
In an olive branch to Brexit supporters, the Home Secretary - who was in the Remain camp - will set out plans for a new government department to take charge of negotiating Britain's exit from the EU to be headed by a senior minister who campaigned for Leave.
A YouGov poll for The Times of 1,000 Conservative Party members put Mrs May ahead on 36% with Mr Johnson on 27%. If it came down to a straight two-horse race between the pair, the advantage for Mrs May was even bigger at 55% to 38%.
The entry into the race of the two candidates seen as the likely front-runners follows announcements by former defence secretary Liam Fox and the self-styled "underdog", Work and Pensions Secretary Stephen Crabb, that they were putting themselves forward.
Health Secretary Jeremy Hunt and Education Secretary Nicky Morgan, who have also both indicated they could stand, have until the close of nominations at midday to make up their minds.
Mrs May will warn that the process of leaving the EU will not be "brief or straightforward", she will stress that Britain with remain an "outward-looking and globally-minded and big-thinking country" that is "open for business and welcoming to foreign talent".
"We need a bold, new, positive vision for the future of our country - a vision of a country that works not for a privileged few but for every one of us," she will say.
"The job now is about uniting the party, uniting the country and negotiating the best possible deal for Britain."
Dr Fox, who campaigned for Leave in the referendum and who ran against Mr Cameron in the 2005 leadership election, will hope to draw support from the right of the party in the battle to succeed him.
The former cabinet minister resigned in 2011 after being found guilty of breaching the ministerial code over his links with self-styled adviser Adam Werritty, whom he met 40 times in the Ministry of Defence and on trips abroad.
Asked what he could do as leader that Mr Johnson could not, he told LBC Radio: "It's a question of experience, it's a question of background in all of this - which of course matters in politics - I think it's a matter of setting out a very clear stall.
"I think that we can't allow the Conservative leadership campaign to be totally dominated by the issues in the referendum.
"There are many other issues that I care very passionately about - as a doctor I care a great deal about what happens to our healthcare in this country, as a former defence secretary I care what happens to our armed forces."
The timetable for the contest - which will end with a new prime minister being named on September 9 - was formally approved by the backbench 1922 Committee.
A series of run-offs will begin on Tuesday as MPs whittle down the hopefuls to two, who will tussle for the support of grassroots members in a one-member-one-vote contest.