Theresa May once pledged to “fight to stop the third runway” at Heathrow and warned it would have a “detrimental” impact on her constituents, it can be revealed.

Newsletters from Mrs May to local voters uncovered by The Sunday Telegraph show she warned Heathrow expansion would “increase noise and pollution”.

It has also emerged that she lobbied the independent airports commissioner while Home Secretary over concerns about the impact of expansion on the environment.

The revelations come just days before Mrs May is expected to proclaim support for a third runway at Heathrow and will cause embarrassment.

They will leave her open to the same claims of backtracking on earlier promises as David Cameron, who was criticised for ditching his “no ifs, no buts” promise to oppose a new runway.