A spokesman said: "Unfortunately it has been necessary to postpone the British Gas Great North Swim, which was due to be staged on Saturday and Sunday, September 4th and 5th, at Windermere.
"This is due to prevalence of blue-green algae and the impact that this may have on safety in the water."
The decision was taken by the event safety group after stringent water quality checks were completed.
The organisers said: "We are looking at a potential new 2010 date for the Great North Swim and will let you know by email on or before Tuesday, September 7."
Such a shame. Hopefully it will still go ahead, but there are bound to be so many fewer participants.
Mind you, after a lifetime of swimming in it, I don't seem to be any worse the ware! Do I ?