A respected policeman from Watford who died after a diving accident saved the life of his colleague moments earlier, an inquest has heard.

Detective Sergeant John Gilbert, a respected child abuse investigator with the Metropolitan Police, realised he and Christopher Read were running out of air so released his friend's heavy weight belt to allow him to rapidly ascend to the surface.

But following him to the surface from a depth of about 30 metres, the 58-year-old suffered severe decompression sickness that led to his death, West Sussex Deputy Coroner Martin Milward heard on Wednesday.

Giving evidence at the inquest, a diving safety expert said: “There is no doubt in my mind that John's actions saved Chris' life.”

Both men were members of the Metropolitan Police Sub Aqua Club (MPSAC) and were diving on a wreck several miles off the coast of Worthing, West Sussex, on August 8 this year.

Mr Read, who had been a friend of the father-of-two for 15 years, told the inquest the pair had joined two other divers and the skipper of the police club's Blyth catamaran Proteus at Littlehampton Marina.

He said they had planned and prepared the wreck dive, which was his first at 28 metres having previously only descended to a depth of 20 metres.

Mr Read, who wore two air tanks during the dive, said: "We were just swimming about, occasionally checking how much air we had.

"We were just swimming around a wreck, he was pointing out conger eels and eventually we gradually worked our way up [to the surface]."

After about 25 minutes, the inquest heard the pair were six metres below the surface when Mr Read, who had swapped from his first tank to his second, found he could no longer breathe in oxygen.

He said: "I tried straight away to swap over to the other cylinder. I looked around for the other regulator (mouth piece) because I spat mine out.”

As Mr Read became increasingly panicked, DS Gilbert passed him his own spare regulator.

But during the underwater emergency, the pair, who were now breathing from the same tank, sank back to the wreck and it was at this point DS Gilbert decided to remove his colleague's weight belt.

This allowed Mr Read to swim to the surface, despite risking decompression sickness, and was a decision that saved his life, an expert witness told the coroner. DS Gilbert, now dangerously low on air himself, followed him up.

Jim Watson, safety and development manager at the British Sub Aqua Club, said: "There is no doubt in my mind that John's actions saved Chris' life.

"He certainly provided something to breathe in a water environment which is very important and there can only be one ending if you stay under water without oxygen."

Skipper Kevin Fletcher-Biles, a serving Metropolitan police officer, said Mr Gilbert was a "careful and prudent" diver and a veteran of more than 350 dives.

Speaking of the moment DS Gilbert resurfaced, he said he heard him shout at first before going quiet.

He said: "He was not making any noise at that stage. I noticed that John's head was back, bobbing under the water and his lips were blue.

"There were no bubbles coming from his mouth."

Mr Fletcher-Biles made a Mayday call that was responded to by a lifeboat and then a coastguard helicopter, which airlifted both injured divers to Worthing General Hospital.

Mr Read made a full recovery but DS Gilbert, from Woodmere Avenue, died the next day from decompression sickness, meaning oxygen bubbles expanded in his lungs and entered his brain.

Recording a narrative verdict, Mr Milward said: "Diving is recognised to be a very enjoyable pursuit and it is also a potentially dangerous one.

"Whilst all care was taken on this occasion there is always a risk that a problem can occur despite all the training.

"I believe the assistance John gave Chris not only at the surface but in making the emergency ascent by discarding his weight belt, those actions by John have undoubtedly saved Chris' life.

"Tragically John has not been so fortunate."

Mr Milward also praised divers Stephen De Burgh-Thomas and Paul Beekan who administered CPR during the tragedy.

Paying tribute to the officer, Detective Superintendent Reg Hooke of the Metropolitan Police's child abuse investigation team (CAIT) said: "Standing 6ft and 3ins tall, John was known among colleagues as their gentle giant.

"He was a hard-working and highly professional member of Edgware CAIT.

"John will be sadly missed by everyone who had the privilege of working alongside him - both within the MPS and outside it."