The Tories have 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."