A US military helicopter that crashed during a training operation in the UK, killing four crew members, was struck by a flock of geese, leaving the pilot and co-pilot unconscious.

The HH-60 Pave Hawk helicopter was assigned to the 56th Rescue Squadron and based at the nearby US air base RAF Lakenheath in eastern England.

It went down in January in marshland near Cley next the Sea on the north Norfolk coast.

The US Air Force issued the findings of its investigation today.

The report said Brigadier General Jon Norman, president of the inquiry's board, had "found clear and convincing evidence that multiple bird strikes caused the mishap by rendering the pilot and co-pilot unconscious and disabling the trim and flight path stabilisation system".

The helicopter had been conducting a training mission to replicate the scenario of rescuing a downed F-16 jet pilot.

It was flying over Cley Marshes when a flock of geese, probably startled by the noise, took off and struck the aircraft.

At least three birds smashed through the windscreen and at least one also hit the nose, the report found.

With the pilots left unconscious and the stabilisation system disabled, the helicopter crashed into the ground about three seconds later.

All those on board: Captain Christopher Stover, Captain Sean Ruane, Technical Sergeant Dale Mathews and Staff Sergeant Afton Ponce, were killed.

The report added as well as the loss of life, the crash cost the US government an estimated $40m.