DAVID CAMERON finally arrived in Downing Street last night after five days of extraordinary wrangling, telling the British public: “Our best days still lie ahead.”

The new premier pledged to lead a “proper and full coalition” with the Liberal Democrats – the first such coalition in Britain since the crisis of World War Two.

Standing outside No.10 in the gloom of 8.45pm, Mr Cameron spoke repeatedly of the “difficult” challenges ahead, tackling deep economic, social and political problems.

And he told the country: “This is going to be hard and difficult work. A coalition will throw up all sorts of challenges.

“But I believe, together, we can provide that strong and stable government that our country needs.”

But Mr Cameron also sought to inspire voters, saying: “I came into politics because I love this country and I think its best days still lie ahead.”

Among the first people to telephone their congratulations was President Barack Obama.

Mr Cameron – at just 43 and with no ministerial experience behind him – becomes the youngest Prime Minister since 1812.

Extraordinarily, as the new premier entered No.10, it was unclear whether he would head a formal coalition with the Lib- Dems, or be propped up in a looser alliance.

The Lib-Con talks finished little more than an hour earlier, apparently reaching a coalition agreement – but one still requiring the assent of both Tory MPs and Lib-Dem MPs and key activists.

There were rumours that the Lib-Dems had been offered no fewer than six Cabinet posts, with Mr Clegg on course to be deputy prime minister.

Within minutes, it was confirmed that George Osborne – Mr Cameron’s closest ally – will be Chancellor, with Vince Cable, the Lib-Dem treasury spokesman, his deputy.

Other Cabinet postings were expected to be William Hague (Foreign Secretary), David Laws (Education), Andrew Lansley (Health) and Liam Fox (Defence).

Mr Cameron was summoned to Buckingham Palace after Lib-Dem talks with Labour fell apart in acrimony – torpedoed by Labour’s own ministers and backbenchers. At 7.20pm, Gordon Brown, with his wife Sarah alongside him, spoke outside No.10 to announce to the world: “It is my intention to tender my resignation to the Queen.”

His voice cracking with emotion, he 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.”

The departing prime minister was then joined by his two young sons, John and Fraser, who walked with their father and Sarah, hand-in-hand up Downing Street, to a waiting Jaguar.

Mr Brown was then driven out of Britain’s most famous street for the last time – heading to Buckingham Palace, to bring down the curtain on 13 years of Labour rule.

Around 50 minutes later, Mr Cameron arrived at Buckingham Palace with his wife Samantha, for an audience that lasted 25 minutes.

The Lib-Con talks dragged on for 5½ hours because of extensive Lib-Dem demands, including immediate fixed-term Parliaments – requiring the coalition to last four years. The document was said to run to 20 pages, with the Lib-Dems thought to have forced the Tories to drop plans for inheritance tax cuts and tax breaks for marriage.