A major offensive to liberate Islamic State’s last stronghold in Iraq will begin within weeks the Defence Secretary has said, as it was disclosed the militants have tried to shoot down RAF jets with surface-to-air missiles.

Tornado and Typhoon jets are carrying out the RAF’s most intensive strike campaign for more than 25 years as they launch scores of attacks to back Iraqi and Kurdish forces preparing to move into Iraq’s second city of Mosul.

British commanders have for the first time disclosed the risks of the air campaign as they admitted Islamic State in Iraq and Levant (Isil) fighters have repeatedly used missiles to try to bring down jets.