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Zac Goldsmith has a “mountain to climb” to avoid defeat in the race to be Mayor after Sadiq Khan opened up an 11-point lead in first preference votes, according to a new poll.

The YouGov survey for the Standard put Conservative Mr Goldsmith on 20 per cent, with Labour’s Mr Khan on 31 per cent. The gap was seven points in March.

A quarter of Londoners are still “don’t knows”, while a further eight per cent “would not vote”.

Pollsters believe many members of these groups — in expressing such a view so close to polling day — are unlikely to vote.

Once “don’t knows” and “would not vote” are excluded, Tooting MP Mr Khan’s first preference lead extends to 16 points, taking into account likelihood to vote.

Ukip’s Peter Whittle is on seven per cent, Green Sian Berry is six per cent, Liberal Democrat Caroline Pidgeon is on five per cent, while Respect’s George Galloway is hardly troubling the scorer.

Mr Khan would be short of a majority on first preferences, on 48 per cent, compared to Mr Goldsmith’s 32 per cent.

The figures almost mirror headline voting intentions in London, with Conservatives on 30 per cent and Labour 46 per cent.

The poll suggests the mayoralty will be decided on second preferences. Once included, without “don’t knows” and “would not vote”, and weighted by likelihood to vote, the Labour contender is on 60 per cent and the Richmond Park MP on 40 per cent.

Laurence Janta-Lipinski, of YouGov, said: “Sadiq is on course for victory, built on his party’s core vote, while Zac Goldsmith is left with a mountain to climb.”

Mr Khan is around three times ahead in inner London and narrowly in front in outer London. More Londoners believe the former Labour minister, rather than backbencher Mr Goldsmith, will improve their quality of life, tackle Islamic extremism, keep Londoners safe, improve policing and modernise the transport network.

The two are almost level-pegging on who will clean up London’s air, while more people say Mr Goldsmith will “cost me more”. He is also trailing over being trustworthy, principled, whether he will deliver on promises and “cares about people like me”, although he is seen as more ambitious.

Mr Khan is seen as more likely to change his mind, after a U-turn on support for Heathrow expansion.

However, Mr Goldsmith is seen by more people as out of touch, divisive, boring and slippery.

YouGov interviewed 1,017 adults in London between April 15 and 19.

Watch our head-to-head debate between Zac Goldsmith and Sadiq Khan live on our Facebook page and London Live from 7pm tonight.