A Norwich triathlon club may be forced to run its flagship event as a duathlon due to blue-green algae in a Norfolk Broad.

In just over a week, scores of triathletes will be descending on Whitlingham County Park, near Trowse, to take part in the annual Norwich Triathlon.

The event, organised by city-based Tri-Anglia and sponsored by Cozens-Hardy Solicitors, is now in its seventh year and will see about 430 competitors take part on July 3. But since the beginning of June, triathlon club members have been unable to undertake their open water swimming training at Whitlingham's Great Broad after reports of a larger than usual bloom of blue-green algae were confirmed.

Some blue-green algae produce toxins that could pose a health risk to people when airborne droplets containing toxins are inhaled while swimming.

Weekly tests of the water quality have been conducted by the Environmental Protection Agency (EPA) who have reported levels approaching 10 times the authorised safety limit.

The results of the final test are due to be submitted on Thursday, June 30, just three days before the triathlon.

In the meantime, entrants are being warned that the triathlon - consisting of swim, bike, run - may be turned into a duathlon where competitors will run, bike, run. Mark Philo, event organiser, said: 'We are working closely with the Whitlingham Outdoor Education Centre, who look after the lake, and are eagerly awaiting the results of the final test results. Pending this result, we will post another email to all competitors along with a club website posting to announce the final decision. This situation is wholly out of our control. We sincerely hope the swim leg will go ahead, but feel it only right and proper that we alert entrants to the very real possibility that it might not.'

If Norwich Triathlon goes ahead as planned, it will cater for all levels of triathlete, with an 'Olympic' distance event (1,500m swim, 40k bike, 10k run) and a 'Sprint' held over half the respective distances. There is also a Relay race, where teams of three, comprising a swimmer, a cyclist and a runner, complete the same Olympic course as the individuals.

Otherwise, if the event is turned into a duathlon, competitors will not swim but instead run a lap of the lake which is about five kilometres before heading out on their bikes and then completing a second run.