Llyn Padarn is a popular tourist visiting spot that dominates Llanberis
People are being warned about a potentially toxic blue-green algal bloom on a lake in Snowdownia.
Environment Agency Wales says users of Llyn Padarn, near Llanberis, in Gwynedd should not undertake sports that would immerse them in the water.
Pet owners and farmers are also advised not to let animals drink the water.
Warning signs have gone up around the the lake but Gwynedd council said the location is still open to the public and can be used for sports.
The algae, anabaena spiroides, has a 50% chance of being toxic and tests are under way to determine the level of potential toxicity.
If the algae is confirmed to be toxic, then it has the potential to be fatal to pets and livestock.
Skin contact with the algae can cause rashes and severe illness if swallowed.
Anyone who has come into contact with water from the lake in the last week and who has become ill is being advised to seek medical advice immediately.
The lake and its environs are still open to the public
Environment Officer Mark Medway said: "People may have noticed slight discolouration on some parts of Llyn Padarn.
"People often report this to us thinking it is pollution, but it is a natural phenomenon.
The best thing to do is not to participate in immersion sports on the lake and to make sure that pets and livestock don't drink the water until test results have been completed."
Gwynedd council maritime and country parks officer Barry Davies said: "Llyn Padarn and Llanberis is still very much open for the public to visit and enjoy.
"We've taken the precaution of positioning warning notices around the lake following advice from the Environment Agency and the [National Public Health Service for Wales].
"We would advice those visiting the area and who intend using the lake for recreational sports to follow the advice provided until further notice."