José Pacheco, the director of the Portuguese Environment Agency (APA), told Lusa News Agency: “A significant density of a species of red algae that could be dangerous to public health was detected on Sunday."

“It was decided, together with the health authority, to advise against bathing in the ocean in the area between Faro Island and Vilamoura.”

He added that the “species in question is still unidentified” but that “samples have been collected for analysis” and the results are expected in the coming days.

It was decided to close the beaches to bathers in the areas where the algae has been found to be most dense.

The APA regional official admitted that other beaches may be affected in the next few hours or days, as the red algae is moving across the Algarve in a westerly direction.

"There is already a concentration of the algae near the beach of Rocha Baixinha, in the municipality of Albufeira".

José Pacheco added that the ban will be maintained until all public health safety conditions are assured: "We do not have any timetable for the situation to be normalised."

A source from the National Maritime Authority told Lusa that the beaches began to raise the red flags on Sunday, the day the warning was issued "to advise against bathing in the sea".