Seven crab fishermen needed hospital treatment after their boat was blasted by a Second World War bomb off the Norfolk coast.

Five crew members on board the Galwad-Y-Mor suffered significant injuries, some of which were life-changing, after the explosion last December 15, the Marine Accident Investigation Branch (MAIB) said.

It added: “Although the physical injuries were significant to five of the seven crew, they were fortunate not to be killed.”

The fishermen were hauling a string of 100 crab pots from the seabed when they noticed “a lot of tension” on the main line, an MAIB report said on Thursday.

The skipper revved the engine in a bid to break the line free, but at about 11.22am three loud bangs resounded on the main deck.

The move had disturbed an unexploded 250kg (550lb) bomb dropped into the sea during the war, the investigation found.

The MAIB report said it was a German-made high-explosive – probably an SC250, a bomb widely used and feared during the Blitz.

The blast triggered shockwaves which sent the boat rocking, the MAIB said.

Power immediately cut out and water began flooding the deck while the skipper roused the night watchman and sent a distress call to the Coastguard.

The wheelhouse was wrecked and the hull plating ruptured, while the engine, gearbox and switchboard were sheared from their mountings.

At 11.48am, a search and rescue helicopter was sent to the accident site, about 22 miles away from Cromer in north Norfolk.

The captain of another nearby vessel, the Esvagt Njord, heard the distress call from around four miles away and sent a rescue boat to help save the crew.

The fishermen were hoisted on board the Esvagt Njord and given first aid, including from a paramedic.

Two were airlifted and taken to hospital at around 1.50pm, while the others were taken to hospital later in the afternoon.

A shard of metal later found in a crab pot was sent to specialists for forensic examination.

Analysis suggested to “a high degree of certainty” that it came from an SC250, the investigation found.

The report said unexploded bombs remains highly volatile, even after many years underwater.

It ruled the crew, whose training, experience and emergency preparedness helped them survive, could not have anticipated what happened.

“The aim of this report is to highlight the dangers that still exist with unexploded ordnance in the seas around the UK, and the actions to take should fisherman encounter any.

“In this case, the skipper and crew could not have foreseen the explosion and their level of preparedness to deal with such an emergency saved lives,” the MAIB added.