A proposed ultra low emission zone around Heathrow would be "virtually unenforceable", an MP has claimed.

Bosses at the airport revealed recently they were looking into creating a zone similar to that which already exists in central London, excluding all but the greenest vehicles.

The measure was part of Heathrow's recently published 10-point action plan to tackle pollution, which exceeds EU limits to the north of the airport.

But Brentford & Isleworth MP Ruth Cadbury this week questioned whether such a scheme, which would require Transport for London's approval, could ever happen.

Speaking on Tuesday (June 9), in a parliamentary debate at Westminster Hall on air pollution, she said the impact of planned public transport improvements on reducing harmful emissions was "not going to be very significant".

"More extreme measures have been suggested," she added. "Clean Air London talks about an ultra low emissions zone around Heathrow, but to be effective that zone would have to be so enormous it would have a serious impact on the economy of the Thames Valley area and would be virtually unenforceable."

She said smog from Heathrow-related traffic on the A4 and M4 was not just a silent killer, causing respiratory illness and morbidity, but was also responsible for the "greasy dirt" found on her constituents' washing, cars and garden furniture.

"It's quite clear that on air quality grounds alone, expansion at Heathrow can't go ahead because of the breaches of EU air quality legislation that would entail," she said.

Heathrow claims the vast majority of traffic on surrounding roads including the M4 is not connected to the airport but says it is taking steps to make the airport greener.

It claims forecasts by the Airports Commission, which is weighing up rival bids for expansion at Heathrow and Gatwick, show EU pollution thresholds could be met even with a third runway.

But chiefs at Gatwick Airport recently claimed a third runway would be "unlawful" as it would increase pollution around the airport.