The decision took effect at 9am on Thursday, with the local coastguard saying that the ban was not likely to be lifted any sooner than 24 hours.
“In the wake of the heavy rains of Tuesday the water overflowed the weirs placed on the streams and brought dirty waters onto the beach," said the coastguard commander for Cascais, Mário Domingues on Thursday.

"As a precaution it was decided to bar the beach to bathing, but we hope that tomorrow [Friday] everything can go back to normal, when we have the results of the analyses", he added.
The tests were carried out by the Portuguese Environment Agency (APA) and Cascais council and, according to reports, came back showing thw water quality was 'excellent'.
The head of the Lisbon and Tagus Valley regional health authority, Mário Durval, told Lusa that the ban had been "absolutely necessary", given that “there were no guarantees that the water was not contaminated”.