Gordon Brown has committed Labour to hold a referendum on changing the voting system "early" in the next Parliament if Labour wins the election.
What is said to be wrong with the existing system?
Critics of the "first past the post" system, where candidates who get the most votes in individual constituencies are elected, say it is unfair and does not reflect the number of votes cast for different parties. They point out that Labour was elected in 2005 despite only getting 35% of all votes cast and the system punishes smaller parties. Reformers say too many votes are effectively wasted in safe seats where either Labour or Conservatives have large, in-built majorities. Results, they say, increasingly hinge on the preferences of a small number of voters in a handful of swing constituencies which is undemocratic.
What is the prime minister proposing?
Mr Brown told the Labour conference the case for changing the voting system to ensure MPs get majority support was "stronger than ever" and that, after this summer's expenses scandal, politics must be more accountable. He said a referendum would be held on the alternative vote (AV) system, a mechanism supported by many Labour politicians. Under this system, voters rank candidates in order of preference and anyone getting more than 50% in the first round is elected. If that doesn't happen, the candidate with the fewest votes is eliminated and their second choices allocated to the remaining candidates. This process continues until there is a winner. While not proportional, this aims to ensure majority backing for those elected.
So could the system change before the next election?
No. This is the start of a process that could take many years to come to any fruition and only if Labour is re-elected. If the Conservatives win the election, any change can be ruled out.
What do critics of the system want?
Historically, most of those seeking electoral reform have urged a form of proportional representation, or PR, where the number of seats a party wins is more closely aligned with the number of votes they get. The Liberal Democrats have long backed this.
Is there a single system which reformers support?
No. Different electoral systems are now in place across the UK, all with their own advocates.
How does it work in Scotland and Wales?
Voting for the Scottish Parliament and Welsh Assembly is done through what is known as an additional member system. Some representatives are elected via the traditional first past the post method but voters get to cast a second vote for "top-up" seats, allocated in proportion to the number of votes.
What about Northern Ireland?
In Northern Ireland local and Assembly elections (and Scottish local elections), voting is done on a single transferable vote basis which sees more than one candidate elected from a single constituency. Voters number candidates in order of preference and all those passing a defined threshold - calculated by dividing the number of valid votes by the number of seats plus one - are elected. Their surplus votes are distributed to other candidates on the basis of other preferences with low-scoring candidates being progressively eliminated.
Are all the systems used proportionate?
No. The Mayor of London and other UK mayors are elected through a system known as the supplementary vote. Voters choose their first and second preferences and a candidate can only be elected in the first round if they get 50% of the vote. If no-one achieves this, all but the top two candidates are eliminated and their second preferences redistributed to the candidates still in the race. The candidate with the most votes is then elected. This is only suitable for electing a single office holder or MP.
Have these issues been considered before?
There have been periodic reviews of the electoral system over the last 20 years, none of which resulted in change. The most significant, a 1998 review headed by former Lib Dem peer Lord Jenkins, argued for a mixed system known as Alternative Vote Top-Up. This would see up to 85% of MPs elected under the alternative vote system but on a nationwide constituency basis. A second vote would be held for the remaining 15% of MPs. They would be elected from a series of county and city lists, taking into account votes cast and the number of MPs already elected in each area. The Blair government did not act on the Jenkins recommendations, despite including a commitment to a referendum on electoral reform in its 1997 manifesto.