Updated 2.30pm

Liz Truss has been elected the new leader of the UK's Conservative Party and will succeed Boris Johnson as Britain's next prime minister.

The foreign secretary comfortably beat her rival, former finance minister Rishi Sunak, by about 57 to 43 per cent after a gruelling summer-long contest decided by just over 170,000 Conservative members -- a tiny sliver of Britain's electorate.

In a short victory speech at the announcement in a central London convention hall, Truss said it was 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. 

Details are expected in the coming days.

Truss will be only the UK's third female prime minister following Theresa May and Margaret Thatcher. 

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

The 47-year-old consistently led 42-year-old Sunak in polling throughout the long leadership campaign that began in July after Johnson announced his departure, following a slew of scandals and resignations from his government.

Postal and online voting closed Friday after eight weeks of campaigning that Truss described to the BBC as "the longest job interview in history".

Truss told the Daily Mail that as prime minister she would "do everything in my power to make sure everyone, no matter where they are from, has the opportunity to go as far as their talent and hard work takes them".

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

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

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

Forty-three per cent said they did not trust her "at all" to deal with the burning issue of the rise in the cost of living.

'Worst in-tray'

Whoever emerges as winner faces "the worst in-tray for a new prime minister since Thatcher", The Sunday Times wrote.

The UK is gripped by its worst cost-of-living crisis in generations, with inflation soaring into double digits and energy prices shooting up on the back of Russia's war in Ukraine. 

Millions say that with bills set to rise by 80 per cent from October - and even higher from January - they face a painful choice between eating and heating this winter, according to surveys. 

"If I'm elected prime minister, I will act immediately on bills and on energy supply," Truss told the BBC on Sunday, while declining to go into details.

British newspapers, including the Times and Daily Telegraph, reported Monday that she was considering freezing energy bills for consumers, with the government reimbursing suppliers.

Truss campaigned on a promise to slash taxes and prioritise economic growth, with Britain tipped to enter recession later this year.

She said Sunday she would "within a month present a full plan for how we are going to reduce taxes" and "get the British economy going".

Sunak vowed further government support to help people pay their energy bills and said curbing inflation would be his priority, attacking Truss's tax-slashing plans as reckless. 

"We shouldn't rule anything out. I mean, we're facing a genuine emergency. I think anyone pretending that that isn't the situation isn't being straight with the country," he told the BBC on Sunday.

Polls show public support for an early general election and the Conservatives face a growing challenge to retain their 12-year grip on power.

How Truss made it to the top

Truss became foreign minister a year ago after holding a series of ministerial posts in departments including education, international trade and justice.

She began her political journey as a teenage member of the centrist Liberal Democrats before switching to the right-wing Conservatives.

In 2016, she campaigned for the UK to remain in the European Union but quickly switched allegiance when Britons backed Brexit.

Her dress sense and love of photo opportunities - posing in a tank in Estonia and wearing a fur hat in Moscow - have earned her comparisons to Tory icon Thatcher.

Her sometimes stiff style has become visibly more relaxed and allies have sought to soften her image, revealing her love of karaoke and socialising.

Truss smiles atop a tank. Photo: UK Government/FlickrTruss smiles atop a tank. Photo: UK Government/Flickr

Highland ceremony

The announcement on Monday sets in motion a chain of events.

On Tuesday, Johnson will deliver a farewell speech at Downing Street.

He will then formally tender his resignation to Queen Elizabeth II, and she will appoint his successor in the so-called kissing of hands ceremony. 

For the first time in her reign, the 96-year-old monarch will appoint the prime minister at her Scottish retreat, Balmoral, rather than at Buckingham Palace in London.

This comes as the queen has suffered mobility problems and been forced to cancel a number of public engagements.

The next prime minister will be the 15th since the queen took the throne.

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