The Government is facing calls for an investigation into allegations that it is colluding with BAA, the airport operator, over plans to build a third runway at Heathrow and allow an extra 500 flights a day over London.

The Department for Transport has secretly passed key information supporting the expansion to the Spanish-owned company six months before it is due to be published in a consultation document.

The department has also allowed senior BAA officials to influence a series of tests designed to show whether the third runway would breach limits on air pollution and noise.

The Times has learnt that BAA has a team of 34 people working with civil servants, influencing the tests so that they find in favour of building the new runway. The department has given BAA a full copy of the preliminary results but is refusing to allow any opponents of the expansion to see them.

Mike Forster, BAA’s head of strategy for Heathrow, admitted at a recent conference that he had seen the results and that they were “encouraging”.

Ferrovial, the Madrid-based construction company that paid £10 billion for BAA last August, is counting on the third runway being approved to make a substantial profit on its investment.

A group of 12 local authorities in and around London that oppose the third runway has written this week to Sir Gus O’Donnell, the Cabinet Secretary, asking him to investigate whether the department has breached the Civil Service code.

Edward Lister, leader of Wandsworth council and spokesman for the group, said: “This looks like a very serious case of collusion between a government department and a private company that stands to gain massively from expanding Heathrow.

“The Government now runs the risk that the results of its air pollution studies will be tainted.

“The air pollution work is being paid for with public money. Giving the results first to BAA gives them an unfair head start on everyone else.”

The Government is planning to publish a consultation document in the autumn that will state that a third runway would, subject to certain conditions, pass the pollution and noise tests.

BAA hopes to submit a planning application for the runway next year and to open it by 2017. A new flight path would be created over Acton, Chiswick and Fulham and up to 700 homes, including eight Grade II listed buildings, would have to be demolished to make way for the runway and a terminal.

A BAA spokesman said that the company was supplying the department with information about options for reducing pollution and noise, such as using more modern aircraft, getting more people to travel to Heathrow by public transport and changing the location and design of the proposed terminal.

He added: “If we had not seen the results we would be operating in the dark. If you don’t see them, how do you know how to change the scenario?”

A department spokesman said: “We are working with BAA on the project. It wouldn’t be sensible or indeed possible to do the work without the expertise of the airport operator.”

John Stewart, chairman of ClearSkies, which campaigns on behalf of people living under Heathrow's flight paths, said: “We were promised an independent study on the pollution issue but it appears to be a one-sided process tipped in favour of BAA’s expansion plans.”

Times Recommends

• The bright side of this winter rain
• Story behind the picture: Snow Leopard, 2008
• Times Charity Appeal: The Place2Be