Record numbers of volunteers turned out this year to help clear litter from the UK’s beaches, in the 25th annual Great British Beach Clean.

But the Marine Conservation Society (MCS), which organises the event, warned there were still shocking amounts of plastic litter on UK beaches, despite a 16% decrease compared with last year. It said the average of 16 glass and plastic bottles and drinks cans retrieved on every 100-metre stretch of beach (330ft) surveyed strengthened the case for the urgent introduction of a deposit return system in all parts of the UK.

The clean-up – held over four days in September – attracted record numbers: just short of 15,000 volunteers, double the number in 2017. In total, 8,550kg (1,346st) of litter was picked up across the whole of the UK.

But although the number of volunteers and events rose, the amount of litter collected actually dropped. Helpers retrieved an average of 600 items of litter on every 100-metre stretch surveyed, down from 717 last year. On average, each stretch contained 189 plastic or polystyrene pieces, 38 plastic caps or lids, 21 cotton bud sticks and 16 plastic bottles and cans.

Lizzie Prior, MCS beach and river clean-up project officer, said the situation was “almost certainly a false dawn” and warned against complacency.

Plastic bottles and other waste washed up on a beach in Cork, Ireland. Photograph: Education Images/UIG via Getty Images

“UK governments must certainly not think the crisis is over and slow down or shelve any much-needed litter legislation,” she said. “Litter levels fluctuate year on year and for the last decade have risen by over 15%. The Great British Beach Clean is just a snapshot of the UK litter story and must be seen in the context of the broader trending picture. There’s more litter in our seas than there has ever been.”

Prior said beach cleaning was now seen as trendy, and more people were joining in. “Picking up litter at the beach is now bang on trend. Social media feeds are filled with people’s pictures of what they find at the coast, on rivers and at sea. Finally beach cleaning is cool and mainstream. So we may have found less litter this year because so many more people are cleaning beaches whenever they can.”

The charity said programmes such as the BBC’s Blue Planet II series and the documentary Drowning in Plastic had highlighted the damage a throwaway culture does to the seas.