The Conservatives tonight offered the Liberal Democrats a referendum on electoral reform in a final bid to secure a coalition that would support David Cameron as prime minister.
Shadow foreign secretary William Hague said there was now an "urgent" choice before the Lib Dems.
"The Liberal Democrats have said to the Conservative Party that they are only prepared to enter into a coalition agreement with a party that will change our electoral system to the alternative vote method of voting," Mr Hague said.
"Now, David Cameron and the shadow cabinet and the Conservative MPs have decided that, although our concentration in all of these negotiations has been on the financial situation, on reducing the deficit, on the improvement of education, on the other great issues facing our country, that in the interests of trying to create a stable, secure government, we will go the extra mile.
"We will offer to the Liberal Democrats in a coalition government the holding of a referendum on the alternative vote system, so that the people of this country can decide what the best electoral system is for the future."
Mr Hague, speaking after a meeting of the Conservative parliamentary party in the House of Commons, said only a Tory-Lib Dem coalition would provide "stable" and "secure" government.
"It is urgent that the country has a new and stable government," he said.
"The choice before the Liberal Democrats ... is whether to go in with the Labour Party in a government that would not be stable or secure, because it would rely on other minor parties for any parliamentary majority at all; that would have a second unelected prime minister in a row - something we believe would be unacceptable to the great majority of people in this country; - and which would impose voting reform without any consultation with the people of the country, something we believe to be profoundly undemocratic.
"Or they can choose to continue their talks with us, to make a coalition with the Conservative Party, which is on offer, in a government that would have a stable and secure parliamentary majority; a majority of 76 in the House of Commons, something highly desirable in our current economic situation; that would have an elected prime minister in David Cameron, the leader who obtained by far the most votes and seats in the General Election held last week; and which would say that any reform of our voting system must be subject to a referendum of the people of this country."
Mr Hague added: "We are absolutely convinced that we should not have another unelected prime minister and we should not change our voting system without a referendum.
"And whatever happens now, and whatever decision the Liberal Democrats make, that is ground on which we will stand."
Mr Hague warned it would be a "great mistake" for the Lib Dems to link-up with Labour given their "apparent attachment to improving democracy".
He said: "It would be wrong to construct a government, which wouldn't be stable, which wouldn't have a prime minister elected by the people of this country and wouldn't be submitting a major constitutional change to a referendum of the country."
Mr Hague said that in a possible referendum, Tory MPs opposed to change in the voting system would be "at liberty" to campaign against it.
The Liberal Democrats were only interested in entering into a "coalition with one party or the other," he added.
Shadow chancellor George Osborne said a weak coalition struggling to achieve a majority was "not in the national interest".
"In good faith we are making an offer to the Liberal Democrats of a strong and stable Government with a considerable parliamentary majority, in coalition, and with a referendum on the alternative voting system," he said.
"The Liberal Democrats want to change our voting system and we are making that offer.
"So the parliamentary party, in the extraordinary meeting where we genuinely consulted them following a meeting of the shadow cabinet, has pretty much to a person - I can't think of a single person who objected - endorsed the position set out and agrees to make this offer now to the Liberal Democrats as a final offer."
Shadow schools secretary Michael Gove, speaking after the meeting of the parliamentary party, said: "Our aim has been to present the Liberal Democrats with a very clear choice.
"A strong, stable government with a clear majority capable of making the right decisions in the national interest versus an unstable government comprised of a variety of parties that cannot take those decisions.
"The public deciding on voting reform versus parliament deciding on voting reform.
"An elected prime minister who the public has seen put through his paces in the leadership debates versus a second unelected prime minister.
"It is for them to decide. It is their choice. This is our offer, they can decide."
Mr Gove said there had been "unanimous" support among Tory MPs and peers present at the meeting.
Asked what the mood in the party was, he replied: "Loyal. Immensely understanding. Appreciative of what David did during the campaign.
"They recognise that David has changed the party in order to help them get to this point and there is trust and respect for the way he has handled this."
The parliamentary party was heard cheering and banging on desks on at least half a dozen occasions during the meeting, which lasted around 50 minutes.
But a couple of backbenchers were seen angrily confronting a member of the shadow cabinet afterwards on the plans to offer the Liberal Democrats a referendum on AV.
Told about Mr Osborne's claims the party had "to a person" endorsed the strategy, one of the backbenchers said: "I wasn't given a chance to speak, so I don't know how he can say I'm happy with it."
The Tories' latest pitch for a deal with the Lib Dems came after the momentum of power-sharing negotiations shifted dramatically late this afternoon.
Earlier, a series of face-to-face meetings between negotiators from the two parties and their leaders appeared to be heading towards an agreement that would install Mr Cameron as prime minister.
But Lib Dem MPs indicated they were unhappy with elements of the package put forward by the Conservatives and Lib Dem leader Nick Clegg requested formal talks with the Labour Party.
Gordon Brown subsequently announced that he was stepping down as Labour leader in what appeared to be an attempt to remove one of the potential obstacles to a Lib-Lab pact.
Mr Clegg hailed the Prime Minister's "difficult personal decision", saying it "could help ensure a smooth transition to the stable government that everyone deserves".