Liz Truss has defeated Rishi Sunak in the Tory leadership race, winning by a convincing margin.
Ms Truss won with 81,326 votes from the party membership, compared to Mr Sunak’s 60,399.
She said it was an "honour" to be announced as the winner, after a “hard-fought campaign” against Mr Sunak.
Ms Truss also said: “I also want to thank our outgoing leader, my friend, Boris Johnson.”
She continued: “Boris, you got Brexit done, you crushed Jeremy Corbyn, you rolled out the vaccine and you stood up to Vladimir Putin. You were admired from Kyiv to Carlisle.”
New Conservative Party leader Liz Truss
HANNAH MCKAY
Liz Truss defeated Rishi Sunak in the Tory leadership race
PA
The result, confirmed by chairman of the 1922 Committee Sir Graham Brady on Monday, marks the beginning of a new era for Britain.
Responding, Mr Johnson said: "Congratulations to @trussliz on her decisive win. 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.
"Now is the time for all Conservatives to get behind her 100 percent."
Mr Sunak added: "Thank you to everyone who voted for me in this campaign.
"I’ve said throughout that the Conservatives are one family. It’s right we now unite behind the new PM, Liz Truss, as she steers the country through difficult times."
Labour leader Sir Keir Starmer tweeted: “I’d like to congratulate our next prime minister Liz Truss as she prepares for office.
“But after 12 years of the Tories all we have to show for it is low wages, high prices, and a Tory cost-of-living crisis.
“Only Labour can deliver the fresh start our country needs.”
And Liberal Democrat leader Sir Ed Davey called for a general election, saying it is "time to scrap the energy price hike".
Ms Truss' first task will be a trip to Balmoral, as the Queen has opted not to travel back to London for the customary Buckingham Palace address with the Prime Minister in waiting.
The Queen will receive Mr Johnson on Tuesday at her Highlands home, where he will formally tender his resignation.
This will be followed by an audience with the new Tory leader, where Ms Truss will be invited to form a government and take over as PM.
It comes after weeks of sometimes bitter in-fighting among allies of the former Chancellor and Foreign Secretary.