Nick Clegg said his party had overwhelmingly endorsed the coalition
Liberal Democrat leader Nick Clegg has said he hopes his party's coalition with the Conservatives will herald a "new kind of government".
He said he would not have agreed to the deal unless convinced it could deliver the changes Lib Dem supporters want.
Five Lib Dems will be in the cabinet, including Mr Clegg as deputy prime minister, and there are expected to be about 20 in government posts in total.
Mr Clegg said the coalition had been "overwhelmingly" endorsed by the party.
Lib Dem MPs and the party's federal executive discussed the agreement at a meeting that concluded just after midnight.
Mr Clegg said: "We are now going to form a new government. More importantly we are going to get a new kind of government. I hope that this is the start of the new politics I always believed in.
"We are politicians, clearly, of different political parties, but I believe we are now united in wishing to tackle the immense challenges this country faces and deliver a fairer, better Britain."
He acknowledged some Lib Dem voters may be unhappy at the deal with the Conservatives, who saw party leader David Cameron installed as prime minister on Tuesday following Gordon Brown's resignation.
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"I am acutely aware that I carry your hopes and your aspirations with me into this coalition agreement," Mr Clegg said.
"I can imagine this evening you will be having many questions, maybe many doubts, about this new governing arrangement.
"But I want to assure you that I wouldn't have entered into this agreement unless I was genuinely convinced that it offers a unique opportunity to deliver the kind of changes you and I believe in.
"So I hope you will keep faith with us, I hope you will now let us prove to you that we can serve you and this country with humility, with fairness at the heart of everything we do and with total dedication to the interests and livelihoods of everyone in Great Britain."
Mr Clegg praised former prime minister Gordon Brown for his "immense dignity, grace and profound sense of his public duty" in the past few days, describing him as a "towering figure" in British politics.
A Lib Dem party spokesman said both the parliamentary party and the federal executive had approved the coalition deal by the three-quarters majority required.
After the meeting, David Laws MP, who was part of the Lib Dems' negotiating team, said the agreement with the Conservatives had "a lot of Liberal Democrat policies in it".
"We're in politics to try to change things for the better and I think that people will see when the agreement is published that is exactly what it does and it delivers on many of the manifesto agreements that we have been campaigning on in the last few weeks," he said.
"This is not just a package of constitutional geekery that means something to politicians in Westminster, but this is a package that will do a great deal for the country."
The Lib Dems will hold a special conference in Birmingham on Saturday for activists to discuss the deal.
The Conservatives and the Liberal Democrats have agreed to the UK's first coalition government in 70 years.
The Tories won the most seats in the general election last week, but not enough to secure an overall Commons majority.
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