A referendum will be held in May on whether to change the system for electing MPs. Here is a guide to the issue.

What will the referendum be about?

The public will be asked 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).

What happens at the moment?

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

How does AV work?

The AV system asks voters to rank candidates in order of preference. People can nominate as many preferences as they like. Only first preference votes are counted initially. Anyone getting more than 50% of these is elected automatically. If that doesn't happen, the candidate with the fewest votes is eliminated and their second choices allocated to the remaining candidates in a second round of counting. If one candidate then has more than 50% of the votes in this round they are elected. If not, the remaining candidate with the fewest votes is eliminated and their second preferences (or third preferences if they were the second choice of someone who voted for the first candidate to be eliminated) reallocated. This continues until one candidate has 50% or more of the vote in that round of counting, or there are no more votes to be distributed.

Can people still vote for just one candidate?

If someone votes for just one one candidate under AV, their vote will be counted once in each round that takes place. But any further preferences they could have stated will not be taken into account.

What do pro-AV campaigners say?

They argue that too many votes are effectively wasted under the current system, with elections decided by a small number of voters in a handful of seats where no single party has a large majority. This discourages people from voting, they say. A key weakness of first-past-the-post, they say, is that two-thirds of MPs are now elected with less than 50% of support of voters and that this undermines democracy and reduces the legitimacy of MPs. They say candidates will have to work harder for votes and reach out to a broader cross-section of the electorate.

And what are the arguments for keeping first-past-the-post?

Anti-AV campaigners say the current system generally leads to stable government and has historically reflected the will of the public, in that unpopular governments have been voted out. They argue that first-past-the-post is straightforward and easy to understand. They say parties get elected on a manifesto and are expected to implement it, while, under other systems more likely to produce indecisive outcomes, the government is decided after the election by horse-trading and political fixes with manifesto pledges being ditched and promises broken.

Where do the leading parties stand?

Conservative Prime Minister David Cameron supports retaining first-past-the-post while his Liberal Democrat deputy Nick Clegg is campaigning for a switch to AV. Their views reflect the positions of most of their MPs and activists. Labour leader Ed Miliband is campaigning for a yes vote in the referendum but his party is split on the issue. Here's a party-by-party guide.

When will the referendum take place?

On 5 May 2011, the same day as elections to the Scottish Parliament, the Welsh Assembly, the Northern Ireland Assembly and local elections in many parts of England.

What question will voters actually be asked?

The question to be put is: "At present, the UK uses the 'first past the post' system to elect MPs to the House of Commons. Should the 'alternative vote' system be used instead'? Yes or no?"

How much is the referendum costing?

The No campaign has said the poll will cost about £90m to stage and that an extra £156m will be incurred in switching to AV. The Yes campaign has accused its rivals of "lies", saying the claims are largely based on the alleged cost of introducing electronic counting machines and that these are not required for AV elections. The government has said holding the referendum on the same day as other elections around the UK will save about £17m.

When will the outcome be known?

The referendum is overseen by the Electoral Commission. Votes will begin to be counted at 1600 BST on 6 May, the day after the poll. The outcome of the referendum is expected to be known later that evening.

Is AV used elsewhere?

Australia is the only major democracy to use the same type of AV system as the one being proposed for the UK, but voting is compulsory there. Papua New Guinea and Fiji also use AV - but most democracies use versions of proportional representation or first-past-the-post.