The UK leadership race narrowed to five on Tuesday as Brexit figurehead Boris Johnson prepared to defend his runaway favourite status in his first television debate.

Johnson expanded his lead over his remaining rivals in a second secret ballot held by 313 lawmakers from the ruling Conservative party in the lower House of Commons.

The former foreign minister and London mayor picked up 126 votes while former Brexit minister Dominic Raab got knocked out after falling three short of the 33-vote threshold.

Moderate international development minister Rory Stewart continued his against-the-odds challenge by expanding his support base from last week's 19 to 37.

Foreign Minister Jeremy Hunt picked up 46 votes and environment minister Michael Gove 41. Interior minister Sajid Javid scraped through with exactly 33.

Britain is frantically searching for a leader after Prime Minister Theresa May stepped down last month as Conservative party chief over her repeated failure to deliver Brexit.

The island nation is now due to split away from the other 27 European Union member states on October 31.

But how and when the chaos paralysing UK politics - and frightening business leaders - ultimately resolves itself may only be decided by who becomes the next prime minister.

Further rounds of voting scheduled for Wednesday and Thursday will whittle the list of contenders down to just two.

The finalists will face the ruling party's 160 000 grassroots members in a vote next month to decide the winner.

Johnson's lead is so substantial that some UK media say the others are simply fighting for a good spot in his future government.

Their attacks on him have been measured and the race itself has been distinguished most by revelations about various contenders' past drug use.

Johnson has partially managed to achieve his success to date by stage-managing his media appearances and skipping Sunday's first TV debate.

He will try to avoid making any big blunders that could put the breaks on his momentum when he makes his debate debut on BBC television Tuesday evening.

Johnson is often a flamboyant speaker who has extensive background as a journalist.

But he has been known to make remarks that outrage many - including by comparing veiled Muslim women to "letter boxes" last year.

He also came under attack this week for reportedly making contradictory promises to various parliamentary factions in private meetings aimed at rallying his support.

UK politicians are split between ardent Euro-skeptics who would prefer to see Britain leave without any strings attached to the bloc.

Moderate Brexit backers want some sort of deal that preserves established economic links between the ancient trading partners.

Johnson has warned that he is prepared to get Britain out of the European Union without an agreement - as long as it is is done by October 31.

But he has also called this tough talk a negotiating tactic designed to scare Brussels and force it to compromise on the nagging issue of status of Irish border.

EU leaders have long ruled out re-opening the binding part of the agreement they all signed with May last year.

French President Emmanuel Macron has been particularly critical of Britain's political stalemate and questioned the wisdom of extending Brexit for a third time.

Only Stewart is openly trying to salvage May's pact in the hope of pushing it through Britain's splintered parliament on the fourth attempt.

The other candidates have all said they would agree to a no-deal divorce if this was the only remaining option for leaving the EU.