A series of hard hitting posters has been produced Enlarge Image

The practice, known as tombstoning, was stopped last year after CCTV captured images of children leaping from a ship's mast into shallow water below.

Now posters are being put up around the waterway warning of the dangers.

Images include one of a boy floating face down in the sea with the slogan: "We'd hate to say we told you so".

The trend for tombstoning - jumping off cliffs, piers, harbour walls or other high points into the sea - has claimed a number of lives around the UK.

Earlier this month father-of-six Delwyn 'Delly' Jones, 46, from Troedyrhiw near Merthyr Tydfil, died when he jumped off a building roof and landed on rocks at Berry Head, Brixham, Devon.

We are trying to get the message across now before something does happen Clare Palmer

The port authority plans to distribute the posters to local schools in its bid to discourage youngsters.

Community officer Clare Palmer said: "It's brilliant to encourage any young person to be outside and to be involved in sport and to enjoy our waterway.

"But they don't realise traffic around the port has increased significantly and will keep on doing so. Also the type of vessels have changed."

She said last summer's footage of a youngster jumping from the mast of a ship had prompted the ban.

"They have been looking for bigger challenges - that involved some of the commercial vessels last year - where does it stop?

"We are trying to get the message across now before something does happen."

But not everyone is convinced the campaign will work as swimming in the port has been a tradition amongst the town's youngsters for many years.

Vicky Hargrave, a youth worker at Project Milford - an internet cafe used by the town's youngsters, said: "They like doing it, they've been doing it and their parents have done it.

"I say to them something is going to happen - don't go - but they still do.

"The posters just seem to encourage them more - a couple have come in and said they are not real.