The additional member system will be used in next year's elections to the Scottish parliament and Welsh assembly.

Under this system, some members are elected for traditional first-past-the-post seats, but others will be returned, in a second vote, in larger constituencies on a party list PR system.

This formula will seek to use the 'top-up' members to make the total assembly composition broadly in line with the votes cast for parties, ironing out any distortions.

The party list system will be used for the first time in the UK in European elections next June.

Tony Blair and his wife Cherie going to vote

The country will be divided into large constituencies and parties draw up lists of people they would like to see elected in them, putting their preferred choices first.

Electors vote for parties, and seats are allocated roughly in proportion to their percentage share. If a party's vote, for example, entitles it to three seats in a constituency, the top three names on its list would be declared elected.

This system is in use in many European countries, including Austria, Belgium, Denmark, Spain, Switzerland and Turkey.

The single transferable vote system was used for elections to the Northern Ireland assembly.

It aims to reduce the 'wasted vote' phenomena under first-past-the post.

This formula allocates a number of MPs to each constituency. The winning post is the quota - the number of valid votes divided by the number of seats plus one.

Electors are asked to number candidates in order of preference. Those passing the quota are declared elected and the number of their votes above the quota distributed to other candidates in accordance with their supporters' lower preferences.

Low-scoring candidates are progressively eliminated and their votes distributed to other candidates in accordance with their supporters' lower preferences.

The alternative vote system will be used to elect the new mayor of London.

Voters pick candidates in order of preference and only those scoring more than half the votes are elected at the first count. Otherwise all but the top two candidates are eliminated and their second preferences redistributed.

In 1987, the Working Party on Electoral Systems recommended the Additional Member System for a Scottish Parliament. It also wanted a regional Party List System for the House of Lords and for elections to the European Parliament.

The group also called for a supplementary vote system for the House of Commons.

The Labour Party accepted the recommendations on all of these except the House of Commons, where it was agreed to commit the party to a referendum.