How the voting system works

The first is for their choice of Mayor. The big names - Ken Livingstone, Boris Johnson and Brian Paddick - have already declared. But other fringe candidates will also come forward. Expect around a dozen names on the ballot paper come 1 May.

The winner gets to serve as Mayor for a fixed four-year term and have power over issues such as transport, policing, the environment and the London Olympics.

The second vote is also for Mayor - a second preference vote. This is cast on the same piece of paper but in a separate column. For example, a voter may wish to give his or her first preference vote to Lib-Dem Mr Paddick and second choice to Labour's Mr Livingstone.

However the second vote only comes into play if a voter's first-choice candidate is eliminated after all the first preference votes are cast. To return to the example above, if the second round results in a Ken v Boris contest, then the vote that was initially cast for Brian Paddick will become a vote for Ken.

The other two votes are for the London Assembly. This has 25 members and meets at City Hall, where its job is to scrutinise the Mayor and examine issues of concern to Londoners. The Mayor is not a member of the assembly but regularly appears in front of it to answer questions.

Fourteen of its members are elected to serve constituencies that typically stretch across two or three London boroughs, such as Enfield and Haringey or Hounslow, Kingston and Richmond.

The other 11 are chosen by their parties under a "top-up" system of proportional representation based on each party's support across London.
Voters will receive two separate ballot forms to elect their "constituency" and "top-up" members.

This means they will be handed three pieces of paper when they enter the polling station. They will be coloured yellow, orange and pink to help distinguish between the different elections. But there remains the potential for confusion.

In the last City Hall elections in 2004, more than half a million votes were lost because voters marked the papers incorrectly. That poll was more complicated because elections were held for the European parliament at the same time. A further change has been made this year - the introduction of two ballot papers for the assembly rather than one.

But the biggest problem could yet be caused in the mayoral vote if electors place their crosses in the same column rather than adjacent columns - making it impossible to work out which candidate is their first and second preference.

With opinion polls indicating that Ken and Boris are neck and neck, voters need to ensure they mark their papers correctly if they want to have an effect on the result.

* For further information and a step-by-step guide on how to vote, click here to visit www.londonelects.org.uk