Conservative leader David Cameron is at Buckingham Palace to accept an invitation from the Queen to form a new government.
He is poised to become Britain's new prime minister after Gordon Brown resigned.
Mr Cameron's Conservative Party won the most seats in the UK general election last week, but not an overall majority.
They have been in days of negotiations with the Lib Dems - who were also negotiating with Labour.
But the Lib Dems said those talks failed because "the Labour Party never took seriously the prospects of forming a progressive, reforming government".
Labour leader Mr Brown stepped down as PM with immediate effect.
It is not yet known if the Tories and Lib Dems have reached a formal agreement on forming a coalition government, or whether the Lib Dems will agree to support the Conservatives in a minority government.
But Mr Brown has tendered his resignation, after Labour talks with the Lib Dems failed to secure a deal that would have kept his party in power and said he wished the next prime minister well.
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In an emotional resignation statement outside Number Ten, Mr Brown thanked his staff, his wife Sarah and their children, who joined the couple as they left for Buckingham Palace.
Mr Brown said it had been "a privilege to serve" adding: "I loved the job not for its prestige, its titles and its ceremony - which I do not love at all. No, I loved the job for its potential to make this country I love fairer, more tolerant, more green, more democratic, more prosperous and more just - truly a greater Britain."
He also paid tribute to the courage of the armed forces, adding: "I will never forget all those who have died in honour and whose families today live in grief."
The Lib Dem and Conservative teams met for hours of negotiations at the Cabinet Office on Tuesday - four days after the UK general election resulted in a hung parliament.
The talks resumed after Lib Dem negotiators met a Labour team, which followed Mr Brown's announcement on Monday that he would step down as Labour leader by September.
But there were signs throughout the afternoon that the two parties - who together would still not command an overall majority in the House of Commons - would not reach a deal.
Several senior Labour figures, including John Reid and David Blunkett, warned against a coalition with the Lib Dems, particularly if the price involved offering them a referendum on changing the voting system to proportional representation.
After Mr Brown announced he would be stepping down and would see if Labour could do a deal with the Liberal Democrats, the Conservatives upped their offer to a promise of a referendum on changing the voting system from existing first past the post system to AV.