Heathrow Airport campaigners have spoken out after councils threatened David Cameron with legal action , should the third runway go ahead.

Four Conservative-run local authorities appointed a legal team, warning that if the Government did not rule out expansion at Heathrow Airport, then legal action will be launched.

The four local authorities are made up of the London Boroughs of Hillingdon , Richmond-upon-Thames, Wandsworth and the Royal Borough of Windsor & Maidenhead.

All these authorities are long-standing opponents of a third runway at Heathrow.

Harrison Grant Solicitors has written to the prime minister on their behalf explaining how "insurmountable environmental problems" would make government backing for a new runway "irrational or otherwise unlawful".

Heathrow villagers have 'had enough'

Stop Heathrow Expansion was founded in 2014 as a successor group to NoTrag, representing residents in the south of Hillingdon whose lives would be directly impacted by Heathrow expansion, and welcomed the letter.

Robert Barnstone, Campaign Co-ordinator, said: "We welcome this timely letter to David Cameron. It serves as an important reminder that he must not break his pledge that a third runway will not go ahead, no ifs or buts".

Christine Taylor, Harlington resident and Stop Heathrow Expansion supporter, said: " This is no idle threat.

"Residents of the Heathrow Villages have had enough – we’ve been fighting this for over 30 years.

"We want to draw an end to the repeated threat of Heathrow expansion on our communities."

'Squandered' taxpayers' money

But Pro-Heathrow expansion group, Back Heathrow, say residents will be "furious" at the threat from councils.

Robert Gray, Back Heathrow Campaign Director, said: "Thousands of taxpayers living in these four boroughs will be furious that councils are spending their money on preparing to sue the Government, rather than on providing vital local services.

"Many residents work at Heathrow and local businesses have strong links with the UK's largest single site employer; they will wonder why taxpayers’ money is being squandered in this way.

"Heathrow is a vital national asset and the government should not be held to ransom by four councils when there is enormous support for expansion from local residents, businesses, communities and MPs across the UK."

READ MORE: The case for and against Heathrow Airport expansion