A referendum on 5 May 2011 will ask the public whether they want to replace the existing first-past-the-post system for electing MPs to Westminster with a method known as the alternative vote (AV).

Under first past the post, the candidate with the most votes in their constituency is elected as the MP.

With the AV method, instead of just voting for one candidate, voters could rank candidates in order of preference, and these decide the outcome where no candidate wins more than 50% of votes cast.