David Cameron was warned by his own policy chief a year ago that the Government was "exposed on Heathrow" because it had no answers to concerns raised over air quality.

A memo, written by senior adviser Camilla Cavendish, raised alarm about a Whitehall air quality plan drafted by the Environment department which was then run by Liz Truss.

The news came after it was revealed that Mrs May will not make a final decision on building a third runway at Heathrow for at least a year.