Londoners back expansion of Gatwick over a third runway at Heathrow , according to the results of a new survey.

A second runway at Gatwick was supported by 44% of those questioned, with 34% plumping for a bigger Heathrow and the remainder undecided.

A total of 1,072 people across the capital were asked their opinions for the poll, with the results published by YouGov last Friday (November 13).

Nearly four fifths of respondents (79%) said air quality was an important factor in the debate over which airport should be allowed to expand.

Gatwick Airport said that on the morning the findings were published, levels of toxic nitrogen dioxide at monitors close to Heathrow were nearly three times the legal limit.

The airport's CEO Stewart Wingate said: "Support for Gatwick has increased in recent months. The reason is simple. Londoners recognise that it is the best choice to balance economic growth with environmental impact. Most importantly after decades of delay it can actually happen."

A Heathrow spokeswoman said: "The results of the first poll undertaken after the Airports Commission made its unanimous and unambiguous recommendation for Heathrow expansion, was published in August.

"Of the 10,000 people polled, more supported Heathrow expansion than opposed it. We think the decision on airports expansion should be made on the basis of rigorous research and analysis, not on a poll of 1,000 people.

"Heathrow takes air quality very seriously and in order to clean up London's air we need to focus on the real sources of pollution, which in the areas mentioned by Gatwick, are caused by road traffic not related to the operation of the airport."

The latest poll coincided with the publication of a open letter to the Government signed by the leaders of Wandsworth, Kingston, Southwark and Croydon councils urging it to back expansion of Gatwick rather than Heathrow.

They wrote: "We believe the choice is simple. Gatwick expansion is achievable and will give the Government the additional capacity and growth it so clearly wants. Heathrow expansion will fail and deliver nothing but more court battles and wasted years.

"The Government should get behind Gatwick."

John Stewart, chairman of anti-Heathrow expansion campaign group HACAN, said: "This poll re-enforces the message that a 3rd runway at Heathrow is simply not popular with people in London. And it sends a clear signal to the Government that it will face formidable opposition if it gives the go-ahead to a new runway."

Bosses at Heathrow earlier this year unveiled a 10-point plan to help reduce air pollution , which included proposals for an ultra-low emission zone to stop all but the cleanest coaches and buses serving the airport.

It has claimed it cannot be blamed entirely for pollution in the area, much of which it says is caused by non airport-related traffic on the M4 and other roads.

The Airports Commission earlier this year recommended expansion of Heathrow over Gatwick, provided conditions were met.

The Government is expected to announce before Christmas which airport it thinks should get a new runway.