Stephen Crabb has become the first senior Tory to launch a campaign for the party leadership - and the keys to Number 10 - by vowing to make curbing immigration a "red line" in Brexit negotiations.

The Work and Pensions Secretary, who backed staying in the European Union during the referendum campaign, said he would work to implement Brexit and there could be no way to "subvert" the process.

He acknowledged he was the "underdog" in a leadership race which is also likely to feature Brexit campaign frontman Boris Johnson and Home Secretary Theresa May, but at a launch event in central London he said the contest should not be a "two-horse race" between the "Boris/Stop Boris" candidates.

Business Secretary Sajid Javid is Mr Crabb's running mate and is expected to become chancellor if the duo succeed in the leadership race.

Read more: Who are the contenders to replace David Cameron?

The pair hope their "blue-collar Conservative" credentials - both came from humble backgrounds - will provide a contrast to Old Etonian Mr Johnson.

The biggest challenge facing David Cameron's successor will be to negotiate the UK's exit from the EU following the historic referendum result.

Mr Crabb said: "Today we face a set of challenges the like of which we have never seen in Britain, a set of problems of almost mind-boggling complexity.

"There is no playbook available that would explain all the manoeuvres and steps that need to be taken to get through this. There is no manual waiting on anyone's shelf to be dusted down that provides instructions on the way forward.

"There is certainly no candidate in this race who can stand here today and provide all the answers."

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.

Read more: David Cameron has quit as Prime Minister after UK votes to leave the EU

"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 called for an end to the Leave and Remain labels in Tory ranks as he set himself up as a candidate to unite a divided party and country.

The Work and Pensions Secretary has the support of Attorney General Jeremy Wright and children's minister Edward Timpson as well as a handful of Tory backbenchers for his leadership campaign.