Paul Dudson was warned not to expect any great discoveries on his first dive - but he resurfaced off Brighton Marina clutching an ancient cannonball.

He had spotted the coconut-sized, 3kg ball within ten minutes of descending on his maiden scuba dive about 50ft from the shore near Kemp Town.

Paul, 15, was diving with his father Phillip, friend Steve Brown and diving instructor Rob Breskal.

Paul, of Wilson Avenue in Whitehawk, Brighton, said: "I'd asked beforehand whether if we found anything, we could keep it but was told I wouldn't find anything. That made me look even harder but I didn't expect to see something so soon.

"I spotted this round ball just resting on a layer of sand, picked it up and swam with it.

"Rob realised it was probably a cannonball, though I hadn't been 100 per cent sure at first.

"It was quite light and easy to pick up and swim around with. Afterwards, everyone was crowding around, wanting to have a look."

Mr Breskal, owner of the Brighton Dive Company, based in College Road, Kemp Town, Brighton, had jokingly urged the group to keep an eye out for treasures.

But he said afterwards: "I only really say that in the initial briefing to make a joke for people.

"In the past seven years I must have dived in that particular area hundreds of times, yet I've never spotted anything like this, only bits of old wood and metal. I was astonished.

"I've had it looked at by local diving club members and we think it must go back hundreds of years.

"It's made of flint so it must be old because they later started using iron cannonballs. It is not bad going for a 15-year-old diving for the very first time."

Mr Breskal, who has been storing the cannonball in his shop, plans to have it looked at by staff at the Booth Museum of Natural History in Brighton.

And Paul, who attends Our Lady Of Sion School in Worthing, intends going diving again this weekend and will be keeping his eyes peeled.