Building a third runway at Heathrow would make it “impossible” to meet legally binding targets on air pollution, according to Lord Smith, head of the Environment Agency.

In a speech tomorrow the government’s own green watchdog will increase pressure on ministers not to approve the expansion.

Smith will tell his agency’s annual conference: “The government has committed itself to rigorous targets to emissions 80% by 2050, which now includereduce CO2 aviation and shipping. It is impossible to see how they would be achieved if the current plans for a third runway go ahead.

“Business needs are important but our commitments on climate change and air pollution are backed by legislation and cannot be ignored.”

Geoff Hoon, the transport secretary, is expected to give the go-ahead next month for a third runway, which could open in 2020. The decision hinges on whether an expanded Heathrow will be able to meet European air quality targets, due to come into force in 2010.

Gordon Brown has said the government will approve the runway only if strict environmental conditions are met but critics, including the Environment Agency, note it is already in breach.

The agency has been at odds with the government over the issue of pollution since the Department for Transport published a consultation document on Heathrow this year. It said it had not fully weighed up the impact on air quality for people living below the flight path and further afield.

Smith said this weekend: “The European Union directive comes into force in 2010. It is important for the environment and the health of the public, but the UK will struggle to meet the targets it sets. How will the further expansion of Heathrow – with its detrimental impact on air quality and emissions – help in meeting a target that it is predicted we will exceed?”

Smith’s comments come as a poll of British business leaders found that 95% believe a runway would “not make much difference” to their business.

The poll, by Continental Research, of small and large businesses, found that 4% thought a third runway would be of benefit while 37% said they would prefer a high-speed rail link from Heathrow to the north of England and Europe.

Theresa Villiers, the Tory frontbench transport spokeswoman, whose party wants to build a high-speed rail network instead of expanding Heathrow, said: “The tide has turned. It is time for the government to join us in saying no to a third runway and yes to making Heathrow better, not bigger.”

It has emerged that the Prince of Wales is also concerned that the environmental impact of expanding the airport is not being taken seriously enough by the government.

Sources close to the prince said he was not only concerned about the impact on carbon emissions of extra flights and traffic to the airport, but was also worried that a larger airport would increase air pollution and noise around Heathrow.

A spokesman for the prince said that he had no plans to express any opinion about Heathrow.

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