The five remaining contenders to be Britain's next prime minister will go head to head in the first of three televised debates on Friday, hoping a good performance will boost their chances in a battle that so far has no clear frontrunner.

An initial field of 11 challengers has been whittled down following two days of votes by lawmakers from the ruling Conservative Party, but no individual has yet emerged as the obvious successor to Boris Johnson who announced he was stepping down following a series of scandals.

While ex-finance minister Rishi Sunak has topped those two votes, he faces stiff competition from foreign minister Liz Truss, who has the backing of a number of senior figures, and junior trade minister Penny Mordaunt, who polls suggest is the most popular with party members who will decide the winner.

Former equalities minister Kemi Badenoch and Tom Tugendhat, chair of parliament's Foreign Affairs Committee, also remain in the running and will hope that a strong showing in the televised debates will inject momentum into their campaigns.

"Tonight is the big event because what this is actually about is not just Conservative members, as important as they are, it's actually about the country at large," Tugendhat told broadcasters.

"What we need to show, what everybody needs to show, is that they're able to communicate Conservative messages strongly and debate hard. Because this isn't actually a knife fight in a phone box. This is about governing the United Kingdom."

Sunak, whose decision to quit the Treasury last week helped trigger a cascade of ministerial resignations that brought down Johnson, remains the favourite among his 358 Conservative parliamentary colleagues.

But his lead over Truss and Mordaunt is slim, and both could overtake him depending on whom lawmakers who backed other candidates choose to support. The battle has become increasingly hostile as the rivals fight to stay in the contest.