The middle-aged man, who has not been named by police, was diving with two friends 11 miles north east of the town when the tragedy happened.

HM Coastguard received the mayday call at around 3pm on Saturday from the group's boat to say that one of their party had come to the surface unconscious.

Hartlepool Royal National Lifeboat Institution's (RNLI) inshore and all-weather lifeboat crews were scrambled alongside the helicopter from RAF Boulmer.

However, the victim – who is not from the Hartlepool area – was pronounced dead by an RNLI doctor, and his body was taken to the lifeboat station.

It is not yet known what led to the diver's death and an investigation has been launched, but police are not treating the death as suspicious.

Inspector Lee Rukin of Hartlepool Police said: "It is not known at this moment what has caused the man's death, and we are carrying out an investigation and assisting the coroner.

"We are not treating it as suspicious."

Coastguard search and rescue manager, Greg Albrighton, said: "It does happen and we get dozens of divers that die at sea each year, but this has brought it closer to home.

"The divers had very comprehensive diving equipment with them, and it just shows how dangerous deep water diving can be.

"At 3.05pm a mayday call was received from a 'dive-boat' reporting that one of the three divers in the water had surfaced and was unconscious.

"The position of the boat was 11 nautical miles north east of Hartlepool. The search and rescue helicopter from RAF Boulmer was sent to the area with lifeboats from Hartlepool RNLI.

"The angling boat Famous was quickly on the scene and the man was put on board while first aid was given.

"The diver was pronounced dead by the RNLI doctor on board the lifeboat, and was brought to Hartlepool Lifeboat Station."