The government faces being sued by Conservative councils over the proposed third runway at Heathrow.

Four authorities close to the west London airport have issued a legal warning to David Cameron, saying that an escalation in the number of flights would be “irrational and unlawful”.

Lawyers acting on behalf of Hillingdon, Richmond upon Thames, Wandsworth and Windsor and Maidenhead councils have told the prime minister that court proceedings will be launched unless he categorically rules out expansion of Heathrow.

A legal letter to No 10 says that “insurmountable environmental problems” around the airport mean it can never be expanded without subjecting residents to excessive pollution and noise, the Times reported.

The government-appointed Airports Commission made a “flawed assessment” of Heathrow’s green credentials when it made a recommendation in favour of a third runway, it said.

It says that Cameron’s previous promise to block expansion had created a “legitimate expectation” among residents that the project would never go ahead.

The authorities have appointed Harrison Grant, the solicitors that led a successful High Court challenge in 2010 against the former Labour government’s attempt to expand Heathrow.

A decision on airport expansion in the south-east was put back before Christmas until the summer with Gatwick vying with Heathrow for the right to build a new runway.