Andrea Leadsom is seen appearing on the BBC's Andrew Marr Show in this photograph received via the BBC in London, Britain July 3, 2016. [Photo/Agencies]

"ZAP"

"Andrea Leadsom offers the zap, the drive, and the determination essential for the next leader of this country," Johnson said in a statement.

"She has specialised in the EU question ... and will be therefore well-placed to help forge a great post-Brexit future for Britain and Europe," he said.

Johnson had been considered one of the favourites to replace Cameron but ruled out a bid last week after Justice Secretary Michael Gove, who had been widely expected to back him for the top job, unexpectedly announced his own candidacy.

Gove said he had changed his mind about his Brexit campaign colleague, as he did not think Johnson could provide leadership.

The other leadership contenders are work and pensions minister Stephen Crabb and former defence minister Liam Fox who is expected the candidate eliminated on Tuesday.

Both Britain's major parties have been hit by division and recriminations since the EU referendum, with opposition Labour Party leader Jeremy Corbyn also seeming set to face a challenge to his position.

He has refused to resign despite mass defections from parliamentarians in his policy team and a no confidence motion that was passed by an overwhelming majority of the party's lawmakers.

The party's deputy leader Tom Watson is due to meet trade union representatives, Labour's financial backers, in a "last throw of the dice" to try and reach a deal over Corbyn's leadership.