The number of balloons littering the UK's beaches rose last year, with campaigners warning jubilee and Olympic celebrations could make the problem worse.

Although overall litter levels fell on the UK's beaches between 2010 and 2011, the number of balloons, which can harm wildlife, rose by 8 per cent, according to the Marine Conservation Society's annual beach litter survey.

The MCS also warned of a growing trend for pet owners to bag up dog mess - and then leave it on the beach. This category increased by 11% between 2010 and 2011 across the UK and by 71% in Scotland, storing up potential health threats to coastal visitors by harbouring bacteria and affecting water quality, the MCS said.

But there was good news in an overall drop of 11 per cent in the amount of litter found during the beachwatch big weekend last September.

Sewage-related debris, which includes sanitary items and cotton buds flushed down the toilet, was down by a third on the previous year, the survey revealed.

Almost 4,500 volunteers took part in the beachwatch big weekend, cleaning 335 beaches over almost 90 miles and collecting more than 2,177 bags of rubbish.

The MCS said litter levels dropped in 2009 from an all-time high in 2008, before rising again in 2010. The charity hopes the latest drop will mark the start of a downward trend.

Despite the reduction in litter discovered, MCS beachwatch officer Lauren David said levels were still too high and raised concerns about balloons.

"With 2012 set to be a year of celebrations from the Queen's Diamond Jubilee to the London Olympics, we really need people to understand why letting go is a bad idea," she said. "There is clear evidence that balloons harm wildlife in the marine environment and we don't want to see 2012 leave a legacy of littering."

She added: "We're delighted that pet owners enjoy dog-friendly beaches and clearly think ahead by carrying poop scoop bags, but we hope our findings will now encourage them to take the bag off the beach and bin it in one of the many receptacles provided for the job."