Liberal Democrat leader Nick Clegg has said the Conservative Party has the "first right" to attempt to form a government in the "national interest".
After an election night which was a "disappointment" to the Lib Dems, Mr Clegg said it was for the Tory leader to prove himself capable of "seeking to govern in the national interest".
Speaking outside his party's headquarters in Westminster, Mr Clegg said there was now a "very fluid political situation" but he believed the party with the most seats had "the first right to seek to govern, either on its own or by reaching out to other parties".
Aides said Mr Clegg had not been in touch with Tory leader David Cameron but indicated that channels of communication were open in Westminster.
The Lib Dem leader said: "I have said that whichever party gets the most votes and the most seats has the first right to seek to govern, either on its own or by reaching out to other parties and I stick to that view.
"I think it is now for the Conservative Party to prove that it is capable of seeking to govern in the national interest."
Mr Clegg was waiting for Mr Cameron to respond to his overture in a statement later today.
"The ball is now firmly in the court of the Conservatives," a senior Lib Dem source said.
Mr Clegg arrived at the Lib Dems' Cowley Street base shortly after 10.40am, describing last night as a "disappointment" for the party.
He said: "Even though more people voted for us than ever before, even though we had a higher proportion of the vote than ever before, it is of course a source of great regret to me that we have lost some really valued friends and colleagues and we have returned to Parliament with fewer MPs than ever before.
"Many, many people during the election campaign were excited about the prospect of doing something different.
"It seems that when they came to vote, many of them in the end decided to stick with what they know best."
Mr Clegg left Cowley Street in a Jaguar to return to his home in Putney but party sources indicated he was expecting a call from Mr Cameron.
One source said the Conservatives "need to talk to other parties and behave responsibly".
Lib Dem MPs and senior peers will hold a private meeting in Westminster tomorrow as part of the process which could allow Mr Clegg to enter into a coalition with either Mr Cameron or Gordon Brown.
The process applies when there is any "substantial proposal which could affect the party's independence of political action".
Under the complex "triple lock" system Mr Clegg needs the support of the parliamentary party's MPs and the Lib Dems' federal executive in order to form a pact.
However unless three-quarters of the members of those bodies back him, a special conference will have to be convened to decide whether to proceed with an agreement.
If less than two-thirds of those present at the conference support the move then the whole membership of the party will be balloted to approve or block any deal.
But a senior Lib Dem source said it would be "inconceivable" to put the question to the whole membership if Mr Clegg had not secured the mandate to cut a deal by that stage of the process.
The Lib Dems have failed to make the electoral breakthrough Mr Clegg had hoped for despite the high profile he has enjoyed since the televised leaders' debates.
An electoral upset which saw Solicitor General Vera Baird deposed in previously rock-solid Labour Redcar was balanced by losses elsewhere, including one of the Lib Dems' most colourful politicians, Lembit Opik, who fell to a Tory swing in Montgomeryshire.