BORIS JOHNSON, the mayor of London, is to lead a legal challenge against the government if, as expected, Gordon Brown grants the go-ahead to an expansion of Heathrow.

A report prepared for the mayor concludes that the proposed third runway would put the health of Londoners at risk and blight communities under the flight path. It says the government should look at creating a new airport in the Thames estuary instead.

The announcement comes as rebel cabinet ministers fight a rearguard action to block the project which would raise the number of flights from 480,000 to 700,000 a year.

An announcement on the runway had been widely predicted for this week. But with the cabinet still at war over the issue, it is unlikely before the end of the month.

An ICM poll, commissioned by Woodnewton Associates, shows that only 18% of the public want a third runway built, while 74% say they would prefer a new high-speed rail network.

The failure of Heathrow to curb noise and carbon emissions, as required under European regulations, will form the basis of any legal challenge.

The internal Greater London Authority report, an advanced draft of which has been seen by The Sunday Times, criticises ministers for downplaying the impact of an extra runway. To be published this week, it claims Heathrow “already exceeds the health-based EU air quality limit values”.

The move to delay the announcement came after a meeting at the margins of the cabinet held in Liverpool on Thursday. Ed Miliband, the climate change secretary, and Hilary Benn, environment secretary, told Brown the environmental safeguards were “insufficiently robust”.

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