Airport operator BAA has pledged to reduce the number of flights that would take off and land at an expanded Heathrow if airlines fail to adopt cleaner and quieter fleets as quickly as expected.

The commitment comes as BAA and the government face fresh allegations that the consultation process for a third runway at Heathrow was flawed and that emissions and noise data was manipulated to strengthen the case for expansion.

The allegations, which are to appear in tonight's BBC Panorama show, centre on evidence that airport operator BAA made up a "virtual" jet for inclusion in the environmental modelling undertaken in the government consultation after it realised the use of existing four-engine planes on the third runway meant it would likely exceed legal limits for noise and pollution.

According to BAA, the new 450-seat "green" jumbo would boast just two engines and by 2030 would account for more flights out of Heathrow than existing four-engine planes such as the Airbus A380.

However, neither Airbus nor Boeing currently plan to build such a plane, with one industry source telling the Sunday Times that "nothing like this is on the drawing board".

"I don't think it's feasible because the size of engines that would be required for this plane to safely take off do not exist and are not under development," he added.

The revelations represent another blow to the government's support for a third runway.

Last week, business secretary John Hutton again hinted at the government's support for expansion, claiming it was ready to take "difficult decisions on airport expansion". However, ministers have been forced to delay the final decision on Heathrow's expansion following protests from local residents, threatened legal action from environmental groups claiming that the official consultation process was flawed and biased, and the publication of a report from the Environment Agency raising concerns that the expansion would lead to the breaching of legally binding environmental standards.

The allegations regarding BAA's "virtual jet" are just the latest in a series of revelations suggesting that some of the data used to make the case for a third runway was skewed to ensure the expanded airport would not breach regulations on noise and air pollution.

Earlier this year, a Sunday Times investigation revealed BAA and the Department for Transport had worked together to alter data featured in the official consultation. For example, the paper found that BAA initially predicted that 20 per cent of planes taking off from Heathrow by 2030 would be four-engine jets, but subsequently cut the figure to 11 per cent and then to six per cent when fears emerged about noise and pollution levels.

It also obtained emails under the freedom of information act that reveal government officials suspected BAA's projections were over optimistic.

However, BAA today defended its role in the consultation process and insisted the "virtual jet" was an accurate prediction of the type of cleaner and more efficient aircraft likely to be in place by 2030.

Speaking to BusinessGreen.com, a spokeswoman for the company said that given improvements in aircraft's environmental performance and commitments by all major aerospace manufacturers to reduce noise levels from new aircraft by 50 per cent on 2000 levels by 2050, the "virtual jet" represented a plausible representation of future aircraft design.

She also added that even if the twin engine 450-seat design was not adopted the third runway was still not expected to result in a breach of environmental regulations. "The Government's environmental modelling shows that even without this aircraft a third runway at Heathrow could meet air quality limits at 702,000 Air Transport Movements in 2020 and meet noise limits at 605,000 ATMs in 2020," she said. "It would therefore be inaccurate to say that the development of this aircraft is necessary for a third runway to meet environmental limits."

BAA also insisted that were cleaner and quieter aircraft not to emerge as quickly as anticipated, the operator would simply reduce the number of flights from the airport.

"The environmental limits set out in the 2003 Air Transport White Paper are absolute and non-negotiable, [and] they set the environmental parameters within which an expanded Heathrow must operate," explained a BAA spokeswoman. "Modelling and forecasting are necessarily uncertain, but can still provide a useful indication of likely future impacts… If, however, slower fleet replacement or slower advancement of new technology means that noise and air quality emissions do not reduce as quickly as predicted, then airport capacity will grow more slowly than predicted."