Britain’s newest and most expensive nuclear submarine collided with an unidentified merchant vessel off Gibraltar during a training exercise on Tuesday, the ministry of defence has admitted. The £1.6bn HMS Ambush was involved in a “glancing collision” while submerged, the MoD said. The submarine suffered some visible external damage but no one was injured. It subsequently headed for Gibraltar for checks. The merchant ship, reportedly a bunkering barge, was not damaged.