The Great North Swim for held in Cumbria has been forced to cancel the event after blue-green algae which could be dangerous was found in the water.
The algae which can cause skin rashes and sickness was found by the Environment Agency in Windermere.
The charity swim which was due to take place over the weekend had almost 9,000 participants.
Nova International (the event organisers), plan to reschedule the swimming race for later in the year.
Nova International have said that safety for participants was their top priority. A spokesman said: “We’re looking at new 2010 dates at the moment and will notify you by email.”
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“We understand that postponing the event will affect a lot of people, but the safety of all swimmers is our priority.”
Competitors have the choice of taking part in the event when a new date is decided, gain entry into next years Great North Swim or they can have a full refund on their entry fee.
The Cumbrian swim which is now in its third year, raises money for charity and helps to boost the local economy.
Last year 6,000 swimmers took part in the one-mile, open water swimming event and an estimated 20,000 people went along to watch and support the swimmers.