BAA, the airport operator, has failed to control pollution levels around Heathrow while pushing for a third runway and more flights.

New figures reveal that last year air quality broke the government’s own pollution target despite initiatives by BAA to curb hazardous emissions. It means European Union legal limits will almost certainly be breached when they are brought in next year.

The annual mean average level of nitrogen dioxide, a key pollutant that can cause respiratory problems, was 52 micrograms per cubed metre, according to data from an air pollution detector near the perimeter fence. The tougher EU legal limit — and the current government target – is 40 micrograms per cubed metre.

The data are a significant setback for Gordon Brown who faces a growing backbench rebellion over the airport’s expansion. A decision is expected on the third runway this month.

Hilary Benn, the environment secretary, said in a Sunday Times interview last month that failing to hit the EU’s pollution targets was “not something that we can contemplate”. He is also concerned about the increase in carbon emissions that will be caused by a runway.

Transport officials and BAA insist their modelling shows they can build a third runway and dramatically increase the number of flights by 2020 without breaching the EU’s legal limits. But the fresh data raise questions about the credibility of that claim.

Five air pollution detectors monitor nitrogen dioxide in the area around Heathrow. Of these, three failed to meet the government’s own target in 2008. In addition to the high reading at the pollution detector near Heathrow’s perimeter fence, a monitoring site near the M4 had an annual mean average of 49 micrograms of nitrogen dioxide per cubed metre, while another site at Hillingdon had an annual mean average of 43.

The lack of progress in restricting the levels of nitrogen dioxide, despite advances in technology, will be worrying for ministers. The Heathrow monitoring site has consistently recorded levels of nitrogen dioxide between 52 and 55 micrograms per cubed metre over the past five years.

The government has already said it intends to apply to the EU for additional time to meet the nitrogen dioxide targets. Ministers hope to comply with the targets by 2015.

Edward Lister, leader of Wandsworth council, said: “These figures prove BAA cannot control air pollution at Heathrow. A third runway or additional flights would increase these breaches and inflict even greater harm on the environment.”

Ministers were due to announce a decision on a third runway last summer, but faced with growing opposition twice deferred the announcement. The cabinet is said to be split.

A BAA spokesman said the company had introduced a wide range of measures to improve air quality, and pollution levels were improving overall.

The spokesman said: “We want to give the public confidence that Heathrow will only expand if the limits on air quality are being met. If, as we have requested, the government appoints an independent assessor, then the public can have confidence that if, for any reason, the limits are not being met, then the number of flights in and out of Heathrow will be limited.”

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