Government officials have worked through the night in an effort to identify what is thought to be reddish algae bloom in sea water off the coast of Scheveningen and Wassenaar at the weekend.

The results of lab tests will be available on Monday morning, news agency ANP said. According to the Telegraaf, the red colour is due to a high concentration of dinoflagellate plankton which can be extremely toxic.

Swimming has been banned in the stretch of North Sea between the resorts of Katwijk and Scheveningen until the exact type has been identified.

A waterways authority spokesman told broadcaster Nos the red bloom smells and can make people a little nauseous. ‘This happens every summer,’ he said. ‘This year it is later than usual because the weather has not been very warm.’

The bloom, if toxic, could have serious implications for local mussel and oyster production, the Telegraaf said.

Thousands of beach goers are expected to head for the Dutch seaside on Monday as temperatures reach 28 degrees along the coast.