MPs will vote next week on whether a referendum will be staged on electoral reform.
Gordon Brown is expected to confirm this plan in a speech later today. If MPs vote in favour of a referendum, then a public vote on replacing the "first past the post" system will take place after the election.
Both the Conservatives and the Liberal Democrats have criticised the plans. The Tories accuse the prime minister of trying to "fiddle" the system, while the Lib Dems say that the "alternative vote" system proposed does not go far enough.
If the idea is approved, a referendum could be held by the end of 2011.