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 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 is 40 micrograms per cubed metre.

The data are a setback for Gordon Brown, who faces a 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".

Transport officials and BAA insist their modelling shows they can build a third runway and 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.