Boris Johnson's bid to become Britain's next prime minister has been bolstered by fresh support from one of his former rivals.

Health Secretary Matt Hancock threw his support behind Johnson on Monday, saying the Brexit-backing former London mayor is the best candidate to unite the party and is able to deliver an agenda, "with the energy that gets stuff done."

Hancock's endorsement followed Johnson's refusal to take part in a televised debate alongside his five rivals Sunday.

Broadcaster Channel 4 left an empty lectern for Johnson, who has a commanding lead after last week's first round of voting by Tory lawmakers in the contest to replace Theresa May.

May stepped down as party leader earlier this month after failing to secure Parliament's approval for her European Union divorce deal.