A second runway at Gatwick is looking less likely after a Government study found air pollution at Heathrow was less serious than originally feared.

The study opens the way for a third runway and a sixth terminal at Heathrow to handle an extra 175,000 flights a year - the Government's favoured option.

The new runway, capable of handling short-haul aircraft such as the Boeing 737 and the Airbus A320, would open between 2015 and 2020. Its development would leave a similar scheme at Gatwick in doubt.

A 2003 White Paper on aviation said expansion at Heathrow could not go ahead if it would cause air quality to fall below the European Union's minimum standard.

Earlier Department for Transport research found that a third runway would expose 35,000 people living in the area to excessive levels of nitrogen dioxide.

The new study, containing results from 18 monitoring sites around Heathrow, shows that all but two were within EU limits and that problems with pollution could be overcome.

At the moment, development at Gatwick is ruled out until 2019. After that a second runway could be built enabling the airport to handle 80 million passengers a year and creating thousands of jobs.

However, that prospect would diminish if a new runway was built at Heathrow, previously thought unlikely because of the pollution it would cause.

Mark Froud, chief executive of Sussex Enterprise, said: "Any report that reduces the likelihood of Gatwick Airport getting the go-ahead for a second runway would be a huge concern to Sussex businesses and devastating for the local economy.

"Expansion at Gatwick will have less of an impact on the environment than at Heathrow and most of the infrastructure that a second runway would require already exists here.

"Demand for travel at Gatwick is also rising at a fast rate with passenger numbers growing and more flights being introduced.

"This growth will come to a standstill in a few years as the airport reaches maximum capacity - unless a second runway is built."

The Department of Transport will use the new research to show how nitrogen dioxide can be reduced sufficiently to allow a third runway at Heathrow.

The Government has said it will make a final decision by next summer. Ian Poll, professor of aerospace engineering at Cranfield University, was appointed to check the study. He said: "The problem of air pollution is manageable and the continued expansion of Heathrow is desirable and technologically feasible."

Professor Poll said the next generation of aircraft engines would produce far less nitrogen dioxide than those flying today.

Mr Froud said the Heathrow study did not include the increase in pollution from extra traffic or the associated congestion