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 said that in a possible referendum, Tory MPs opposed to change in the voting system would be "at liberty" to campaign against it.