Swimmers and sailors have been assured an unpleasant slick on the surface of the sea at Brighton is not raw sewage.

The foul smelling, creamy-brown substance is in fact rotting seaweed, which is perfectly normal but has occurred in greater quantities than usual this year.

Ian Chapman, Brighton and Hove City Council's seafront officer, said: "I am getting a lot of people ringing me up about the sewage in the water all along the coast - but it isn't sewage.

"It is an algal bloom which happens every year. As soon as the sea gets warm, the seaweed begins to spawn.

"We are left with a bloom of left-over seaweed, which is basically a natural pollution."

The pollution is greatest about 50 metres from the shore line.

Mr Chapman added: "It will disperse if there is any rough weather but it usually goes after a month or so anyway."

Marcus Saint, of Southwick, said the sludge appeared to stretch from the shoreline out for 200 metres and along the coast for several miles.

He said: "Past the power station at Shoreham there is no problem at all. But it stretches from there right to the Marina. It might be safe, but from an aesthetic point of view it is dreadful."