Former British finance minister Rishi Sunak and UK Foreign Secretary Liz Truss have sped up the race in the final contest to become the United Kingdom's next prime minister.

Truss is up against Sunak in a contest to court the 200,000 members of the Conservative Party who will vote to choose the country's new prime minister. Members of the ruling Conservative Party will vote for a successor over the summer, with an announcement due on Sept 5.

Sunak's resignation helped trigger a revolt, which saw Prime Minister Boris Johnson agree to step down following a series of scandals.

Sunak led in all rounds of voting among party lawmakers to reduce the field to two candidates.

However, Truss held a 24-point lead over Sunak in a YouGov poll of Conservative Party members published on Thursday.

On Saturday, Sunak described himself as the underdog in the contest, Reuters reported.

"Be in no doubt, I am the underdog," Sunak said in a speech in the central England town of Grantham, the birthplace of former prime minister Margaret Thatcher. "The forces that want this to be a coronation for the other candidate, but I think members want a choice, and they are prepared to listen."

Truss would be Britain's third female prime minister after Thatcher and Theresa May, while Sunak would be the country's first leader of Indian origin.

Tax cuts at stake

The campaigning focus has been on pledges, or non-pledges, to cut taxes at a time when many people are struggling, as well as defense spending and energy policy.

In his speech, Sunak laid out what he called "common sense Thatcherism", promising careful management of the economy before tax cuts.

But Truss said tax cuts are needed to stimulate growth.

"It is wrong to take money from people that we don't need to take when people across the country are struggling with the cost of living crisis," she told reporters in Kent in southeast England after meeting party members.

In an interview with The Times on Saturday, Sunak said he would put the government on a crisis footing upon taking office.

Britain's relationship with Europe remains of great concern to the Conservative Party membership, generally characterized as more euroskeptic than the wider population, Reuters reported. Hoping to tap into that, Truss, who campaigned for "remain" in the 2016 referendum but is now seen as the heir to Johnson's pro-Brexit position, promised to purge all remaining European Union laws from the statute books by next year.

On Saturday, the English Channel port of Dover battled to clear a chronic backlog of summer getaway delays, which Britain blamed on France but others said was caused by Brexit.

French lawmaker Pierre-Henri Dumont said this weekend's travel chaos would happen again, calling it "an aftermath of Brexit".