Prime Minister David Cameron today said his new coalition with the Liberal Democrats would be united behind the "one key purpose" of giving the country strong and stable leadership for the long term.
With his new Deputy Prime Minister Nick Clegg standing alongside him in the Downing Street garden, he said that their alliance represented a "historic and seismic shift" in the political landscape.
"It will be administration united behind three key principles: freedom, fairness and responsibility," he told their first joint news conference.
"And it will be an administration united behind one key purpose and that is to give our country the strong and stable and determined leadership that we need for the long-term."
Mr Cameron said the appointment of six Liberal Democrats, including Mr Clegg, to the Cabinet was "a sign of the strength and depth of the coalition and our sincere determination to work together constructively to make this coalition work in our national interest".
"We have a shared agenda and a shared resolve to tackle the challenges our country faced: to safeguard our national security and support our troops abroad, to tackle the debt crisis, to repair our broken political system and to build a stronger society."
In a swipe at the outgoing Labour administration, he said rising unemployment figures revealed this morning were "another sign of the human cost of the economic mistakes of the past decade".
He added: "We understand that we are not going to beat these problems overnight and in particular no government in modern times has ever been left with such a terrible economic inheritance."
Under Labour, the country had suffered "a chronic short-termism in government", he said, claiming the five-year power-sharing deal would allow long-term decisions.
He said the coalition is ushering in a "new politics, where the national interest is more important than the party interest, where co-operation wins out over confrontation, where compromise, give and take, reasonable, civilised, grown-up behaviour is not a sign of weakness but a sign of strength".
Mr Cameron said it was a "remarkable and very welcome day".
"Our Liberal-Conservative government will take Britain in a historic new direction, a direction of hope and unity, conviction and common purpose," he said.
"I'm delighted to be standing here with the new Deputy Prime Minister, the two of us together leading this historic Liberal Democrat-Conservative administration."
Speaking after him, Mr Clegg said the country needed the kind of government he and Mr Cameron had come together to provide.
"We have just been through an election campaign and now we have a coalition," he said.
"Until today we were rivals and now we are colleagues. And that says a lot about the scale of the new politics which is now beginning to unfold.
"This is a new government, and it's a new kind of government, a radical, reforming government where it needs to be and a source of reassurance and stability at a time of great uncertainty in our country too."
Mr Clegg said there were challenges on the economy and public finances, with a foreign conflict that "requires resolution", and with a society "still scarred by too much unfairness and inequality".
"At a time of such enormous difficulties our country needed a strong and stable government," he said.
"It needed an ambitious government determined to work relentlessly for a better future.
"And that is what we have come together in this coalition to provide."
Mr Clegg admitted there would be some "bumps and scrapes" over the course of the arrangement but insisted the parties had a "common purpose".
He said: "This is a Government that will last. Not because of a list of policies, important though they are; not because it will be easy, there will be bumps and scrapes along the way, we are different parties and we have different ideas.
"This is a Government that will last despite those differences because we are united by a common purpose for the job we want to do together in the next five years.
"Our ambition is simple and yet profound. Our ambition is to put real power and opportunity into the hands of people, families and communities to change their lives and our country for the better.
"For me, that's what liberalism is all about: ensuring that everybody has the chance, no matter who they are, where they are from, to be the person they want to be, to live the life they want to live."
There would be "clean, open, plural politics" and a "bold, reforming Government that puts fairness back into Britain".
Mr Clegg said "fine words" on the environment would be turned into action.
"I came into politics to change politics and to change Britain for good. Together that job starts today," he added.
Mr Cameron said the two party leaders had examined a minority Tory administration backed with a "confidence and supply" understanding from the Lib Dems but concluded that was "so uninspiring".
The Prime Minister said: "We want to give the country good government, we want to sort out the problems of the debt and the deficit and the problems in public services."
Mr Clegg said: "People last Thursday in the ballot box told us politicians that they didn't think any party deserved an outright majority.
"Yet at the same time it's obvious that we need stability, so the only way you can create stability is by creating a government, a coalition government, which lasts."
He said the Government was "underpinned by a common purpose" to "restore stability to our economy but also giving power back to people".
Mr Cameron said Mr Clegg's responsibilities as Deputy Prime Minister would include political reform.
"Many of the questions have been raised about fixed-term parliaments and that will be his responsibility," he said.
He said he expected that they would be working closely together.
"He has the Deputy Prime Minister's office in the Cabinet Office. We haven't yet explored all each others' offices, but it is pretty close together. This is not going to be a partnership where we have to book meetings," he said.