People and dogs are being warned to keep out of the lake at Pugneys Country Park while tests are carried out on green algae floating in the water.
The lake near Wakefield was closed last week after sunshine and still weather conditions caused high levels of the algae to form. A canoe and kayak club session planned for Saturday was cancelled due to the “toxic algae growth”.
A warning sign erected by Wakefield Council said: “Green algae can be harmful to humans and animals.
“We expect this issue to resolve itself naturally, however please keep yourself and animals under your control out of the water.”
Council bosses said dog warning was a precautionary measure and the rest of the park remained open.