Prime minister Keir Starmer has promised to lead a government that "serves" the whole nation in a speech outside 10 Downing Street.
Starmer, who won a landslide victory at the general election to become the UK's 58th prime minister, said he would get going "immediately" to restore public trust in politics and "rebuild" Britain.
"If you voted Labour yesterday, we will carry the responsibility of your trust as we rebuild our country," said Starmer.
"But whether you voted Labour or not – in fact, especially if you did not – I say to you directly: my government will serve you. Politics can be a force for good.
"We will show that we have changed the Labour Party to return into service, and that is how we will govern. Country first, party second."
He said for too long successive governments had entered office and then ignored the people whose lives they had pledged to improve. "Not this time," he said.
"Changing a country is not like flicking a switch," said Starmer.
"The world is now a more volatile place. This will take a while. But have no doubt that the work of change begins immediately.
"Have no doubt that we will rebuild Britain. With wealth created in every community. Our NHS back on its feet facing the future. Secure borders, safer streets. Everyone treated with dignity and respect at work. Opportunity of green British power cutting your energy bills for good.
"And brick by brick, we will rebuild the infrastructure of opportunity. The world-class schools and colleges. The affordable homes that I know are the ingredients of hope for working people. The security that working class families like mine can build their lives around."
He paid tribute to the "dedication and hard work" of Rishi Sunak, who resigned as Prime Minister earlier on Friday after the Tory party's crushing election defeat, and said being Britain's first Asian prime minister must "not be underestimated by anyone".
Starmer is just the second Labour leader to win a general election in the last 50 years, and the seventh Labour PM ever.
Before delivering his speech on Downing Street, he travelled to Buckingham Palace to formally accept King Charles' invitation to form a government.
The Labour Party secured a historic victory at Thursday's election, returning 412 seats compared to the Tories' 121 at the time of writing.
Adam Payne is editor of PoliticsHome, where this story first appeared