At risk : visitors like seals can eat deadly debris

A total of 150 beaches were surveyed, 17 of them Welsh and for every kilometre surveyed, researchers found an average of 2,800 items of litter.

Although the figure was a drop on last year's total, it was still way above the UK average of 1,700 per kilometre.

The highest concentration of litter was found at Southerndown in the Vale of Glamorgan.

Tourists are to blame for the majority of the litter, but the nation also suffers from the highest concentration of debris left by fishing and shipping vessels.

Wales has always prided itself on the cleanliness of its coastlines with 22 of its beaches holding Blue Flag status - the top European standard for cleanliness and facilities.

But the MCS said it was particularly concerned that plastic pellets left by industrial craft could be eaten by marine wildlife, exposing them to poisonous substances.

Litter: Tourists are believed to be to blame

It said raw industrial plastic pellets could act as magnets for toxic chemicals and their resemblence to natural foodstuffs meant they could be easily eaten by mistake.

They could damage the immune and reproductive systems in fish, marine mammals and seabirds.

Beachwatch co-ordinator Amy Hinks said litter levels were still "unacceptably high" and more than 55% of litter recorded during was made of plastic.

"New evidence about the association of toxic substances with industrial plastic pellets gives us an even greater cause for concern, as plastic fragments from degrading litter could potentially act in the same way," she said.

"There is already a wealth of evidence indicating plastic fragments are regularly ingested by marine life potentially exposing them to toxins such as PCBs."