IT would be the first major shake-up in living memory to the way Britain elects its MPs.

On May 5, the same day as the local elections, voters will be asked to give a "yes" or "no" to the idea of replacing the traditional "first past the post" system of voting with an "alternative vote" system.

It has not been the kind of debate that lends itself easily to glamorous photo opportunities.

But the Yes to Av campaign has been able to cite the support of Oscar-winning actors Colin Firth and Helena Bonham Carter and comedians Eddie Izzard and John Cleese.

Those who have come out against AV officially include fertility expert and Child Of Our Time presenter Lord Winston, while comedian Dara O'Briain has also defended the first past the post system.

What is the choice before the public on May 5?

Each voter will be asked to put a cross against "yes" or "no" in response to the following question: 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?

What is "first past the post" all about?

Each parliamentary seat has one MP and whoever gets the most votes wins - whether they beat the nearest runner-up by 20,000 votes or one vote.

What would the voter have to do differently at general elections under AV?

There would still be only one MP per seat, but instead of putting a cross against one name, the voter would be able to rank candidates in order of preference: "1" next to the first choice, "2" next to the second and so on until you chose to stop. You could vote for as many or as few candidates as you liked.

How will the votes be counted?

The number 1 votes for each candidate are totted up first. If a candidate receives more than half the votes cast, he or she is elected outright.

If no one wins more than half the first preferences, there is a second round of counting. At this stage, the candidate who came last is eliminated and their votes are re-distributed to the candidates who had a "2" put next to them on those ballot papers.

If there is still no candidate with more than half the votes, there is another round of counting - and so on until there is a clear winner.

How do the political parties line up on the issue?

Ironically, the only party that was promising an AV referendum at last year's general election was Labour.

The Conservatives are in favour of keeping first past the post, while the Liberal Democrats wanted a system of full proportional representation (or PR), where the number of seats awarded to each party in parliament directly reflects the number of votes cast.

A referendum on AV was agreed as a compromise under last year's coalition agreement.

Labour leader Ed Miliband is in favour of a yes vote, in common with many in his party, but a number of senior Labour figures are opposed - as is former SDP leader David Owen, who argues that anything less than full PR is a distraction.

What's the argument for changing the system?

Supporters of AV argue that today's system means that your vote counts much more if you happen to live in one of the relatively few seats that are likely to change hands at a general election.

They say people who want to support smaller parties often feel they have to vote for another party which has a better chance of winning in their area.

Under AV, those voters would be able to put a "1" next to their first choice and lend their second preference to another party.

Supporters say the current system encourages apathy among voters and lazy behaviour by MPs, since many of them are elected with a minority of the vote.

Under AV, all MPs would be able to claim support from more than half their constituents.

What's the argument for sticking with the current system?

Supporters of first past the post say the system has served Britain well, delivering clear majorities for a governing party in almost all general elections.

They say a change would be unnecessarily costly.

No to AV campaigners say AV would be unfair, because the candidate winning the most votes in the first round could well end up losing.

They say the system would lead to more coalitions and that this could mean government policy being hammered out in backroom deals, rather than by politicians winning a clear mandate for a manifesto.

Some people who support full proportional representation are also opposing AV, saying it will not produce governments which fully reflect the votes cast nationwide, and that the campaign is a distraction from the need for full PR.

When will we know the result of the referendum?

May 5 also sees local government elections in much of the country, as well as elections to the Scottish parliament and the assemblies in Wales and Northern Ireland.

Referendum votes will be set aside that night for counting on the Friday.