High levels of sewage-related debris also remains unacceptable, according to the report. The debris is made up of tampons, panty liners and condoms but is mostly comprised of cotton buds.
The survey is based on data collected by 4,000 volunteers on 358 beaches in the UK and over 370,000 items were removed as part of the MCS' anti-litter campaign.
Beach visitors contributed the majority of the litter on Welsh beaches and the density of 739 items per kilometre is the highest ratio for any country. Fishing debris was the second most common source of litter, followed by sewage-related debris.
Gill Bell, Welsh officer for the MCS, said that Welsh beaches were a "national treasure" yet the report shows they have more litter than any other UK region.
"No-one will go back to a litter-strewn beach and Welsh beaches appear to be suffering from a rising tide of litter," Ms Bell said.
"Litter is on the increase and it is up to everyone to take responsibility. Everyone must dispose of waste carefully and recycle where possible and before you use the toilet as a wet bin – remember where your cotton bud sticks can end up."
Cotton buds are particularly persistent on UK beaches due to the plastic preserving them in the sea for many years.
Calum Duncan, Scottish conservation manager at the MCS, reiterated the appeal to not use a toilet as a wet dustbin.
"Bag it and bin it - never flush it," he said.
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