Greenpeace has now backed Hillingdon Council's plans to take legal action over the proposed third runway development at Heathrow Airport .

A decision on expanding either Heathrow or Gatwick airport is expected to by made by the Government by the end of the month.

But four local authorities that surround Heathrow, including Hillingdon , have threatened to take legal action if it is allowed to expand.

A pro-expansion campaign group, Back Heathrow, has called the plans for legal action a waste of taxpayers' money.

Greenpeace however, now backing the councils' calls for a judicial review, says the environmental impacts of expanding Heathrow would be unlawful.

'Air pollution time bomb'

Executive director of Greenpeace UK, John Sauven, said: “A third runway at Heathrow would be an air pollution and carbon time bomb.

“It would jeopardise the government's chances of meeting legally-binding air pollution and climate targets designed to protect the health and security of millions of people.

“If ministers are willing to trample over these fundamental laws, we're ready to take them to court to stop them.”

The councils say other claimants could join their alliance in the coming days as the decision looms closer.

Authorities planning legal action are Hillingdon, Richmond , Wandsworth, Windsor and Maidenhead.

Harrison Grant Solicitors have been instructed to prepare a legal strategy, and Greenpeace will be sharing the costs and "bringing new environmental expertise to the partnership".

Airports reaching full capacity

According to the Airport Commission, lead by Sir Howard Davies and published in 2015, Heathrow and Gatwick are both reaching full capacity.

The Commission favoured a new northwest runway at Heathrow because it would allow flights to up to 40 new destinations and create more than 70,000 new jobs by 2050.

But expanding Heathrow to include a third runway would result in the demolition of 783 homes.

The leader of Hillingdon Council, Councillor Ray Puddifoot said: “A new runway at Heathrow will make already illegal air pollution levels around the airport worse and give Heathrow a noise pollution record worse that the top five European airports put together.

“We urge the Government to consider Gatwick as the most realistic and sustainable option, and one which can be delivered at a much reduced cost to the taxpayer and with far less damage to the environment and wellbeing of people."

'Taxpayers money wasted'

Campaign director of Back Heathrow, Rob Gray, said: “Local taxpayers will be angry that their money is being wasted on lawyers and unnecessary legal services instead of on vital public services.

“These councils should not be squandering money allocated to them by hardworking taxpayers, many of whom are proud of the jobs and investment that Heathrow brings to the community.”

Mr Gray added: “Greenpeace can spend its money how it likes but it is ironic that it should be backing four councils that actively support Gatwick expansion when Greenpeace is supposed to oppose building any new runways anywhere.”

Getwestlondon has approached Heathrow Airport for comment .