Comment: race is proving a real test for pollsters

This figure is up on last week's seven per cent Johnson lead and, if repeated in Thursday's election, would see the Conservative heading for City Hall. Second preference voting has stabilised, with little apparent difference between Johnson and Ken Livingstone.

The last five YouGov polls have seen second preference voters gradually deserting Livingstone, Johnson and Liberal Democrat Brian Paddick.

People have increasingly been saying they will use their second vote for other candidates. It is possible the campaign has put voters off. This is not surprising given the low-octane nature of candidates' policies and their performances in TV debates.

Other opinion polling has continued to show small leads for Livingstone.

A Mori poll for trade union Unison had Livingstone three per cent ahead on certain-to-vote first preferences and leading by four per cent with second preferences factored in.

Yesterday, Mruk Cello in the Sunday Times showed Livingstone ahead by 44 per cent to 43 on first preference votes, or by 51 to 49 when second preferences were included. Both Mori and Mruk Cello show fascinatingly high rates of voters who say they are "10 out of 10" certain to vote. Mori is showing 61 per cent and Mruk Cello 64 per cent. Because the mayoral race has been tightly fought, turnout will probably rise on Thursday.

But it is most unlikely that more than 60 per cent of people will actually vote. The first two mayoral contests managed turnouts of 33.7 and 35.9 per cent respectively.

What is crucial to Livingstone and Johnson is the relative shift of first preference votes between them as certainty to vote declines from more than 60 per cent to, say, a more plausible 40 to 45 per cent.

On balance, such a reduction could be expected to favour the Conservative. Livingstone needs to deliver the highest possible turnout to have his best chance of winning. In all the polls, Livingstone is still outperforming the Labour Party's dreadful national position. Mori's work would suggest a Livingstone swing of six or seven per cent from the Tories to Labour. YouGov's polling suggests Livingstone is worth a four per cent swing. For such a longstanding figure, this is remarkable.

It is probably safest to assume this is a close race. The pollsters will explain they are testing opinion when they do their work, not making a forecast. If one or other candidate were to win easily, one or more of the polling firms will need to review their methods. By Friday evening, we will know which.