Former defence secretary Liam Fox will run to become the Conservative Party’s next leader and prime minister.
Dr Fox becomes the latest senior Tory to throw his hat into the ring after Cabinet minister Stephen Crabb launched his campaign to replace David Cameron in Number 10.
Favourites Boris Johnson and Home Secretary Theresa May are yet to formally enter the race but are building support among Tory MPs and ministers.
Sources close to Dr Fox confirmed that he would stand for the leadership again, having lost to Mr Cameron in 2005.
Dr Fox 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.
The Leave supporter will hope to draw support from the right of the party in the race to replace the Prime Minister, who announced his intention to quit in the wake of the EU referendum defeat.
Mr Crabb vowed to make curbing immigration a “red line” in Brexit negotiations if he takes the keys to Number 10 as he set out a vision to unite the deeply divided party and country following the vote to leave the EU.
The Work and Pensions Secretary, whose running mate is Business Secretary Sajid Javid, acknowledged he was the “underdog” but said the contest should not be a “two-horse race” between the “Boris/Stop Boris” candidates.
Meanwhile senior backbencher David Davis, a former leadership contender, threw his weight behind Mr Johnson.
“The biggest issue in front of us for the next several years is going to be managing Brexit, bringing about the improvement in our trade position, the control of our borders – all of those things,” he told the BBC’s Daily Politics.
“That needs vision, optimism, energy, drive – Boris has got them.”
Mr Davis said Mr Johnson would enjoy the support of large numbers of Tory MPs:
“I would be amazed if he is not already passing 100.”
But Mr Crabb and Mr Javid positioned themselves as the “blue collar” alternatives to Old Etonian Mr Johnson.
Mr Crabb highlighted his childhood in Wales where he had a “fabulous education at a really good comprehensive school across the road from the council house where I lived”.
He also delivered a swipe at the former mayor over Mr Johnson’s previous comments on the leadership: “On the rainy rugby fields of West Wales I learned that it is not a question of waiting for the ball to pop out of the back of the scrum. If you want it, you do what’s required.”
Mr Crabb said there could be no “stepping back” from the referendum result and ruled out a second ballot.
Instead, he set out three aims for the negotiations: “One: controlling immigration, and for us this is a red line. The one message that came through louder than any other in the vote last week is that the British people want to control immigration.
“Two: it is vital that we seek to achieve as close an economic relation with the EU as we have now.
“Three: the end of the supremacy of EU law.
“No-one pretends that this will be anything other than very difficult, but these are the three driving principles that give us the best chance of delivering on the instructions given to us.
“Brexit needs to do what it says on the tin.”
Mr Crabb indicated he would not call a snap election if he won: “The answer to the question of instability is not to create further uncertainty.”
In the Commons, former minister Sir Alan Duncan took a swipe at Mr Johnson by appearing to compare him to former Italian leader Silvio Berlusconi.
With Mrs May expected to run against the former London mayor, at Commons question time Sir Alan asked Mr Cameron to “compare the undemonstrative competence and dignity of Angela Merkel with the theatrical and comical antics of Silvio Boris-coni”.
Mr Cameron insisted the Tory race was “an election that I will stay firmly out of”.
Foreign Secretary Philip Hammond said the next leader did not have to come from the Leave camp but must be able to show a “clear” plan for Brexit.
He told BBC Radio 4’s The World At One programme: “Of course it could go to somebody like Theresa May. Theresa May is one of the leading contenders in the leadership competition.”
