An Ironman event due to take place on Sunday has cancelled the swim leg of the triathlon due to water safety concerns.
Organisers Ironman Leeds confirmed that tests of Waterloo Lake in Roundhay Park found blue-green algae levels were "significantly above the threshold" for swimming safely, attributing this to "recent hot weather across the UK".
The event will continue as a bike-run event with athletes taking part in a 112-mile (180km) ride across the Yorkshire Dales and a 26.2-mile (42.2km) run, ending back in Roundhay Park.
Organisers added they recognised "this is not the news athletes want to hear ahead of race day" but "remain committed to putting on an epic day of racing".
In a statement on social media, Ironman Leeds said: "In conjunction with Leeds City Council and following the advice of the UK Health Security Agency Environmental Hazards Team, we are disappointed to announce that today's swim practice and the swim portion of Sunday's race will not take place.
"The recent hot weather across the UK has led to a bloom of blue-green algae, which can be hazardous to humans.
"Test results from Waterloo Lake confirmed the current blue-green algae levels are significantly above the threshold for safely proceeding with the swim."
The swim leg of the race was scheduled to start at 6:00 BST, but the race will now begin from transition with a rolling TT bike start at 07:00 instead.
Athletes will be contacted with more information about race start times, organisers added.
There will still be a number of road closures in place for the cycling and running sections of the event.
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