The most important election in the UK conducted using a version of the alternative vote is the London mayoral contest.
An elected London mayor was one of the promises on which the Labour Party won the 1997 general election, and the promise was made good by the 1999 Greater London Authority Act.
There have since been three London mayor elections – the first in 2000, when Ken Livingstone won as an independent, the second in 2004 when he retained his position as official Labour candidate, and the third in 2008, when the Tory Boris Johnson won. The next London mayor election is in 2012, and Livingstone will again be the Labour candidate.
It's subtly different from what's proposed for the Commons
The version of AV used for London mayoral elections is the supplementary vote, under which voters can choose only first and second preferences. If one candidate does not win more than 50 per cent of first preferences, second preferences are counted for the top two candidates and added to their first-preference tally.
How it has worked in practice
In the first mayoral election in 2000, Livingstone won 39 per cent of first preferences and the Tory Steve Norris won 27 per cent. The second-preferences for Livingstone and Norris were then counted, giving Livingstone a 58 per cent to 42 per cent victory.
In 2004, Livingstone won 36 per cent of first preferences to Norris’s 28 per cent, winning after second preferences were distributed by 55 per cent to 45 per cent.
In 2008, Johnson took 43 per cent of first preferences to Livingstone’s 36 per cent, winning by 53 per cent to 47 per cent after second preferences were distributed.
Very few relevant lessons
It is difficult to draw conclusions from London’s mayoral elections on the possible use of AV for Westminster elections. The position of London mayor is by definition a one-off: there really isn’t a proportional system for electing a single representative.
Moreover, the version of AV proposed for House of Commons is not the supplementary vote but one in which voters can rank all candidates: it would be possible for candidates placed third or even fourth on first preferences under the proposed AV system for the Commons to win seats, which can’t happen with a supplementary vote system.
And, more important, the London mayoral election of 2000 was a freak. Livingstone stood as an independent after being carved out of the official Labour selection process. The official Labour candidate, Frank Dobson, came a poor third, and most of his first-preference supporters gave Livingstone their second preferences.
So – few relevant lessons here. The system we use to elect a London mayor and the one we use for parliament are two separate issues.