A vast slick of scum crept across the waves on the hottest day of the year so far.

The Environment Agency said the sludge, which prompted dozens of calls yesterday, was not sewage.

The slick was caused by common marine algae known as May bloom.

Despite its name, it usually starts to form later in the summer.

An early appearance can be caused by high rainfall, which washes fertilisers from fields into inshore waters, followed by warm, sunny weather.

It forms a creamy-brown foam in the water, which turns darker brown and lumpy as it decays.

This looks similar to raw sewage.

As it breaks down, it also smells of rotten eggs or vegetables. However, it is not toxic.

Visitors to Brighton beach yesterday were shocked by the sludge's sudden appearance.

Kevin Fogarty, 60, who lives near The Level, Brighton, said: "I was walking along the parade and saw this awful-looking brown scum.

"It started about half a mile out between the piers and began bubbling up.

"I didn't go near enough to smell it but it was coming up to the beach line.

"I've not seen this sort of thing since the bad old days."

A Brighton and Hove City Council spokesman said: "There's a lot of this along the South Coast.

"It's a natural phenomenon and lasts about four weeks.

"It looks nasty but is harmless and tends to get broken up in windy conditions."

Jeanette Collett, of the Environment Agency, said: "Every year we get reports of sewage slicks on inshore coastal waters and, of course, we treat each one seriously and carry out tests.

"But almost always, despite its unpleasant appearance, it turns out to be this common May bloom."

She added: "Some marine algae species can present a health risk to marine life as well as humans. These are less common and in some cases cause discolouration of the water, known as red tides."