Runners in a Welsh marathon face disqualification if they are caught dropping litter during the race.
The organisers of Wales' largest running event, the Conwy Half Marathon, said new measures had been implemented to offset the environmental impact of the race.
The event organisers said plastic waste was becoming an "increasing problem" and they would be cracking down hard on litterers during Sunday’s edition.
The Run Wales website said runners would also be "taken off the results if seen discarding their rubbish outside of a water stop or not with a marshal".
According to the BBC, around 3,000 runners took part, although it was unclear if any were excluded under the new regulations.
Following last year’s London Marathon, Westminster City Council collected 5,200kg of rubbish and 3,500kg of recycling - including about 47,000 plastic bottles - from the streets.
For the 2019 marathon, organisers reduced the total number of drink stations on the running route, used compostable cups and provided edible seaweed capsules.
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This year’s Cardiff Half Marathon saw competitors given smaller water bottles and medals made of recycled zinc.
"Half-marathons have become the fastest growing race in the UK, and attract tens of thousands of runners and spectators," said Dr Andrea Collins, of Cardiff University.
"They can have economic benefits for cities, but their environmental impact is becoming increasingly hard to ignore and organisers need to act now if they are to continue staging them in the future.”