A man has told of the terrifying underwater 'scramble' before his friend drowned while shipwreck diving in the North Sea.

Christopher Vanstone, of Brixton, drowned after getting into difficulty around 17 miles off the coast of Lowestoft.

He had taken out his mouthpiece, though the reason why remains a mystery, and then the mouthpiece for his back-up oxygen supply fell by his side where he could not reach it, an inquest heard today.

His dive buddy Neil Cope tried to help him with his own back-up supply, then fought to save him by giving CPR for 40 minutes after hauling him onto a boat while an emergency helicopter was scrambled.

Mr Vanstone, 49, was pronounced dead at the James Paget University Hospital in Gorleston on August 30 last year.

Coroner Jacqueline Lake recorded the death as an accident.

Mr Vanstone's friend Mr Cope had been on more than 150 dives with him and told the inquest Mr Vanstone was an 'experienced' diver who was 'calm under pressure'.

Both men had been using 'complicated' breathing apparatus called a rebreather - which does not release carbon dioxide bubbles into the water as it works on a closed loop.

Mr Cope said Mr Vanstone was 'fully familiar' with how it worked, and was in the habit of checking his equipment before each dive.

Around half a dozen experienced divers were on the third day of a shipwreck diving trip when events took a tragic turn.

A line was placed from their boat, the Raider 5, to the shipwreck on the seabed and Mr Vanstone started to descend along it.

But he suddenly stopped while 28m below the surface.

Mr Cope, who was descending after Mr Vanstone, saw he had taken out his rebreather mouthpiece and the mouthpiece for his back-up oxygen supply was 'dangling' from his hip.

After a struggle he got his own back-up mouthpiece into Mr Vanstone's mouth and he appeared to be breathing.

But when he bent down to reach Mr Vanstone's back-up mouthpiece, the hose of his own back-up was pulled taut and 'pinged' away from Mr Vanstone.

This was recorded in video footage from a camera on Mr Cope's diving suit.

Mr Cope described the moments as a 'scramble' and 'panic' and added: 'Realising there was no hope at depth I decided to go to the surface as fast as possible.'

He flagged the attention of their boat's skipper, hauled Mr Vanstone aboard and performed CPR until a rescue helicopter arrived.

He assured Mr Vanstone's mother Susan Izod that helicopter was the quickest way to get him to hospital, and his state had not changed during CPR.

George Buxton, the boat's skipper, said he saw Mr Cope shouting and waving his arms in the water and Mr Vanstone horizontal and face down next to him.

He added the pair did all they could to try to save Mr Vanstone.

Technical diving expert John Ingle analysed Mr Vanstone's breathing kit, and said while the electronics had flooded it appeared to be working.

Coroner Jacqueline Lake said Mr Vanstone was an 'enthusiastic, competent, qualified diver' with seven years' experience.

She added the reason Mr Vanstone removed his rebreather mouthpiece remains a mystery.

The conclusion was one of accidental death, and Mrs Lake praised Mr Cope for his efforts to save his friend.

'You worked very quickly in very difficult circumstances when I think it would have been very easy to panic,' she said. 'You reacted calmly and did what was best in the circumstances.'

A police investigation found nothing untoward, and a post-mortem showed Mr Vanstone had drowned.