LEGAL action could be launched by the Royal Borough unless a third runway at Heathrow Airport is ruled out, solicitors have warned.

Representatives of the council, alongside, Hillingdon, Richmond and Wandsworth, have written to David Cameron, setting out why he cannot lawfully say yes to the expansion - and have threatened court action.

Harrison Grant Solicitors, who led the councils’ successful 2010 High Court challenge against the plans, have highlighted 'insurmountable environmental problems' in their letter, which would make a Government decision to support Heathrow expansion 'irrational or otherwise unlawful'.

It warns Mr Cameron that the recommendations of the Airports Commission, which favoured building a third runway, 'were based on a flawed assessment of the impacts on air quality and noise'.

According to Harrison Grant, the commission’s conclusions on air quality 'were based upon an error of law' and proposals for reducing noise impacts rely on 'mitigation measures which were speculative and have been wholly rejected by the industry as unworkable'.

The airport's expansion would result in 200,000 extra low flying aircraft over London and the Home Counties each year.

Using the World Health Organisation’s noise impact measure, 766,100 people live within Heathrow’s existing noise footprint, the highest rate in Europe and three times higher than second place Frankfurt Airport with 239,000. 

The letter states that the third runway is more 'damaging' than the previous plan as the runway is longer by 1,300m and suitable for use by all aircraft, the number of predicted air traffic movements is 38,000 higher and more people would be affected by noise.

The letters also highlights that “to date, the Government’s approach to consultation has been selective, inadequate and incomplete.” And that full and thorough consultation is a legal requirement which has to be met before a national policy statement on aviation is made.

Cllr Burbage, leader of the Royal Borough of Windsor and Maidenhead, said: "The Commission’s work on air pollution and noise has been discredited and cannot be used to form a robust national aviation policy. The simple fact is that Heathrow expansion would be too noisy, polluting and damaging to local communities.

"It’s time for ministers to recognise the huge weight of evidence that shows that the third runway plan is impossible.”