David Cameron became Britain's new prime minister on Tuesday and vowed to form a "strong and stable" coalition, after breaking five days of deadlock following an inconclusive general election.
Ending 13 years in opposition for his Conservative party, Cameron was invited to form a government by Queen Elizabeth II after the resignation of Labour premier Gordon Brown.
Speaking shortly afterwards outside 10 Downing Street, he said he would form a "full" coalition government with the Liberal Democrats, who emerged as kingmakers from last week's cliffhanger elections.
"I aim to form a proper and full coalition between the Conservatives and the Liberal Democrats," said 43-year-old Cameron, Britain's youngest premier for some two centuries, accompanied by his pregnant wife Samantha.
"I believe that is the right way to provide this country with the strong, the stable, the good and decent government that I think we need so badly".
One of Cameron's first duties as prime minister will be to take a call from US President Barack Obama, who was expected to telephone imminently, the White House said.
The dramatic confirmation of Britain's new leader came after days of uncertainty in the wake of Thursday's general election, which produced no clear winner for the first time since 1974.
The country now faces being ruled by its first coalition government since World War II.
Brown had announced he was quitting just an hour-and-a-half before Cameron walked through the famous 10 Downing Street front door.
He wished Cameron well as he departed from top-level politics, while acknowledging the personal weaknesses -- such as poor presentational skills and impatience -- which hampered his three-year premiership.
"Only those who have held the office of prime minister can understand the full weight of its responsibilities and its great capacity for good," Brown said.
"I have been privileged to learn much about the very best in human nature and a fair amount too about its frailties -- including my own."
Brown then walked down Downing Street for the last time holding hands with his wife Sarah and their two young sons John and Fraser, who were making an extremely rare public appearance.
Immediately afterwards, he and Sarah were driven from Downing Street to Buckingham Palace for Brown to offer his resignation to the Queen, which was accepted in a 15-minute meeting.
Brown had said he would resign as Labour leader Monday but could have stayed on for several months as a caretaker had Labour struck a deal with the Lib Dems.
He has spoken to his predecessor Tony Blair by phone, media reports said. Labour deputy leader Harriet Harman will act as caretaker leader while a leadership campaign takes place which is expected to conclude by September.
Some 40 minutes later, Cameron arrived at the palace with his wife for a ceremony with the Queen known as the kissing of hands. A statement from Buckingham Palace confirmed he had been asked to form a government.
They left for Downing Street after 25 minutes, their chauffeur-driven Jaguar car attracting toots and waves from passing vehicles, and were greeted by cheering crowds.
In last Thursday's general election, the Conservatives won 306 seats in the 650-member House of Commons -- 20 short of a clear majority of 326 -- followed by Labour on 258 and the Lib Dems on 57.
After five days of talks between Nick Clegg's Lib Dems and Tories -- and briefly between the Lib Dems and Labour -- the two parties have struck a deal but this must now be approved by their lawmakers.
Earlier Tuesday, Cameron had piled pressure on Clegg's party to decide which way to jump after he offered a referendum on their touchstone issue of electoral reform.
"It's now I believe decision time, decision time for the Liberal Democrats," said Cameron.
A deal between the centre-right Conservatives and centrist Lib Dems would be seen as an unlikely alliance.
They have strongly differing views on issues like Europe, defence and immigration but between them have enough to secure a majority in the House of Commons which Labour and the Lib Dems, seen as more natural bedfellows, did not.