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Britain has been plunged into uncertainty over its future government, as both Labour and the Conservatives competed for the support of the Liberal Democrats to form an administration.
Nick Clegg has held talks with David Cameron on the Tory leader's "big, open and comprehensive offer" to Lib Dems, which could see the third party provide ministers in a coalition Cabinet.
Liberal Democrat sources said the two men had agreed they should "explore further" plans for economic and political reform.
But Gordon Brown - who remains Prime Minister until the resolution of the impasse caused by Thursday's inconclusive General Election - made clear that he was ready to deliver immediate legislation for a referendum on the Lib Dems' cherished goal of electoral reform if Mr Clegg signs up to a deal to keep him in Downing Street.
A dramatic day of offer and counter-offer was set in train by an election which produced the UK's first hung Parliament since 1974. With a single seat left to declare, Conservatives had secured 306 MPs in the new House of Commons - an increase of 97 in their parliamentary representation, but 20 short of the 326 threshold for an outright majority. Labour were on 258, after losing 91 seats, and Liberal Democrats were down five on 57.
Despite admitting "disappointment" at his party's failure to translate the surge of support it enjoyed after the televised leaders' debates into votes and seats, Mr Clegg was thrust into the role of kingmaker.
He put the ball firmly in Mr Cameron's court by declaring that, as the party with most seats and votes, the Conservatives had the "first right" to seek to form a government. He challenged them to show themselves "capable of seeking to govern in the national interest". But he was subjected to determined wooing from Labour, with first senior ministers like Lord Mandelson and Peter Hain making clear the party's readiness to offer a deal on electoral reform, and then Mr Brown himself making a direct overture.
In a statement outside 10 Downing Street, Mr Brown committed himself to immediate legislation for a referendum on a "fairer voting system" - with the public to decide what that system should be.
Less than an hour later, Mr Cameron made his own offer, including an all-party committee of inquiry on political and electoral reform to look at the possibility of changing Westminster's first-past-the-post voting system.
This led to telephone talks between Mr Cameron and Mr Clegg, understood to have lasted about 10 minutes, after which a Lib Dem spokesman said: "They agreed that they should explore further proposals for a programme of economic and political reform."
