Conservative rivals Rishi Sunak and Liz Truss will duel in the coming weeks to become the next prime minister after the Tory party's lawmakers held the last vote Wednesday.

Former Finance Minister Sunak, running on a centrist platform of fiscal rectitude allied with "green levies" to fight climate change, again headed the field with 137 votes in the fifth and final elimination ballot.

The crucial race for second place was narrowly won by Foreign Secretary Liz Truss on 113 votes, against 105 for former Defence Secretary Penny Mordaunt.

Sunak and Truss now take their case to Conservative party members, who will decide the new leader and prime minister after a series of nationwide hustings in August. The result will be announced on September 5. 

Sunak's resignation as Finance Minister this month helped to topple outgoing leader Boris Johnson after months of scandal including "Partygate".

"Hasta la vista, baby!"

At his last session of Prime Minister's Questions in the House of Commons earlier Wednesday, Johnson bowed out by saying "Hasta la vista, baby!"

In a hint of support for Truss' low-tax platform, Johnson urged his successor to "cut taxes and deregulate where you can to make this the greatest place to live and invest."

Truss tweeted that she was "ready to hit the ground running from day one."

But whoever wins the Tory race, "like some household detergent, would wipe the floor" with the main opposition Labour party, Johnson predicted.

"Tax cuts, deregulation and tough reform"

Mordaunt lapsed to outsider status after Truss' fellow right-winger Kemi Badenoch was eliminated on Tuesday.

Truss wrote in Wednesday's Daily Telegraph that her plan to revive the economy would be "based around tax cuts, deregulation and tough reform."

A YouGov poll published before the vote indicated that, despite his popularity with colleagues, Sunak was the least appealing candidate to the members. 

But Sunak's popularity with the Tory grassroots has faded since questions were raised over his family's tax arrangements, and as he presided over sky-rocketing inflation, which hit a new 40-year high of 9.4% in June.

In a new policy announcement, Sunak vowed an "ambitious new plan to make the UK energy independent" by 2045 in order to prevent future energy-driven inflation spikes, after the war in Ukraine sent gas prices rocketing.

Mordaunt had headed the same YouGov poll of Tory members previously.

But she slipped after a damaging few days in which her former boss, one-time UK Brexit pointman David Frost, slammed her work ethics.

Johnson announced on July 7 that he was quitting as Conservative leader after a government rebellion in protest at his scandal-hit administration.

It is noteworthy that under Britain's parliamentary system, the leader of the biggest party is the prime minister and can be changed mid-term without having to call a general election.