Stephen Crabb is to launch a bid for the Conservative leadership - on a joint ticket with Sajid Javid as his number two.
The Work and Pensions Secretary and the Business Secretary have teamed up in what Crabb dubbed a "blue collar ticket" to take on favourites Boris Johnson and Theresa May.
Crabb is the first Cabinet minister to declare his intent to succeed David Cameron as prime minister and lead the negotiations of Britain's exit from the European Union.
The timetable for the contest looks set to be extended by a week - with Cameron's successor now due to be unveiled on September 9.
Meanwhile, sources close to Johnson, who fronted the successful campaign to leave the EU, indicated that he would not call a snap election if he succeeded in the race to replace Cameron.
There has been speculation in Westminster that the new leader might go to the country to get a personal mandate to lead the country, although no election is required until 2020.
Crabb has lined up Attorney General Jeremy Wright to act as campaign manager, with a formal announcement of his bid expected on Wednesday morning.
Hugely popular among colleagues, Crabb comes from the sort of ordinary background that chimes with many voters - and Javid is the son of a Pakistani bus driver who became a highly successful investment banker. Both men backed the Remain side although Mr Javid faced claims he was privately in favour of leaving the EU.
Nominations for the contest are expected to open on Wednesday - with May, the Home Secretary, and Johnson, the leading Brexit campaigner and ex-London mayor, vying to be favourite with the bookies.
Health Secretary Jeremy Hunt has said he is "seriously considering" a bid, as has former defence secretary Liam Fox, while Education Secretary Nicky Morgan is also rumoured to be thinking about entering the race.
Whoever ends up in Number 10 will begin extracting the UK from the EU, after Cameron said he would not initiate the process before handing over the reins despite pressure from Brussels for a swift departure.
Chancellor George Osborne has ruled himself out, saying it was clear he could not provide the unity the party needed as the EU referendum had left him a divisive figure.
Asked if someone from the pro-EU side could win, he told BBC Radio 4´s Today: "Absolutely. I am not backing any candidate at the moment but of course I was full-throttled in arguing for remaining in the EU and because half my party wanted to leave the EU I don't think I can be the person who can bring the party together."
Whoever takes the leadership of the party - and the keys to Number 10 - will have to wait a week longer than previously thought.
The executive of the 1922 backbench committee had recommended that it be concluded by September 2 "at the latest" but the Party Board argued that more time was required.