Beach litter mostly from inland, Marine Conservation Society claims
Litter levels on Welsh beaches will only fall if people cut the amount of rubbish they drop on the street or flush down drains, campaigners say.
A Marine Conservation Society (MCS) survey found the amount of litter on Welsh beaches in 2012 had risen by almost half compared to 2011.
It estimates that 80% of the rubbish on shorelines comes from inland sources and is blown or washed to the coast.
Spokesman Robert Keirle said: "We need to update our anti-litter messages."
The figures come after more than 400 volunteers cleaned more than five miles (8km) of shoreline on 45 beaches across Wales as part of the annual Beachwatch Big Weekend last September.
The 2012 clean-up, held in more than 170 other countries and regions worldwide, collected 23,362 items of litter in Wales, filling more than 220 bags.
The survey recorded almost 2,726 pieces of litter per kilometre (0.6 miles) in Wales, a 48% rise on 2011 which recorded 1,839 items per kilometre.
However, more than 3,000 items per kilometre were collected in 2009 and 2010.
Various forms and sizes of plastic made up nearly three-quarters of the litter collected, the MCS said, with the amount in Wales almost 10% higher than the average for the UK.
The society said although Wales had seen a rise in litter washed in by the Gulf Stream and by the prevailing south westerly winds the scale of the litter on Welsh beaches suggested people were "becoming less bothered about littering".
Dr Keirle, MCS pollution programme manager, said items found on beaches such as toilet brush heads and fridge magnets were examples of beach litter that had not been left behind by seaside visitors.
He said: "If you flush a cotton bud stick down the toilet in, say Llandrindod Wells, you may think that that is a long way from a beach.
"But that goes into the sewage system, it is then ends up in a sewage treatment works and from there can flow out to the sea.
"If people are dropping it on the street it can be picked up by the wind or washed by the rain into a drain and into a river and out to the sea.
"We are a much more throwaway society than we were even 30 years ago.
"We need to update the anti-littering message for the new generation to take account of this."