Britain's new Conservative leader Liz Truss on Monday vowed "bold" action to fix the country's worst economic crisis in decades, as she was confirmed as Prime Minister Boris Johnson's successor after a gruelling party contest.

The foreign secretary resisted pressure for a politically perilous early election to confirm her new mandate, vowing instead to "deliver a great victory for the Conservative party in 2024".

Truss beat her rival, former finance minister Rishi Sunak, by about 57 to 43 percent after a summer-long contest decided by just over 170,000 Conservative members -- a tiny sliver of Britain's electorate.

Sunak -- who has been lukewarm about whether he would serve in Truss's cabinet -- tweeted that now was the time for the party to unite as "one family".

But Truss ignored her applauding rival as she marched up to the stage of the central London convention hall, calling it an "honour" to be elected after undergoing "one of the longest job interviews in history".

"I campaigned as a conservative, and I will govern as a conservative," she said, touting Tory values of low taxes and personal responsibility.

Truss vowed a "bold plan" to address tax cuts and the energy crisis, which she has indicated will be financed by much higher borrowing -- even at the risk of stoking double-digit inflation.

Truss, 47, will be the UK's third woman to become prime minister following Theresa May and Margaret Thatcher.

She will formally take office on Tuesday, after Johnson tenders his resignation to Queen Elizabeth II.

"I know she has the right plan to tackle the cost-of-living crisis, unite our party and continue the great work of uniting and levelling up our country," Johnson tweeted.

"Now is the time for all Conservatives to get behind her 100 per cent."

The leadership contest began in July after Johnson announced his departure following a slew of scandals and resignations from his government, including Sunak's.

Truss reserved a portion of her short speech to praising Johnson's record, including on Brexit and the Covid pandemic, and said he was "admired from Kyiv to Carlisle".

That won applause from the Tory faithful present. The right-wing ideologue, however, faces a tough task in winning over public opinion.

A YouGov poll in late August found 52 percent thought Truss would make a "poor" or "terrible" prime minister.

Forty-three percent said they did not trust her "at all" to deal with the crisis in the cost of living, as energy prices rocket amid Russia's war in Ukraine.

Truss is also viewed with mistrust among some of Britain's allies, after taking a hard line against the European Union over post-Brexit trading rules for Northern Ireland.

"I look forward to a constructive relationship, in full respect of our agreements," European Commission president Ursula von der Leyen said, flagging climate change and Ukraine as areas for cooperation. (AFP)