Environment experts say they are mystified as to why Worthing's bathing water failed a top-level quality test.

Tests held during the summer by the Environment Agency revealed the sea had fallen short of European water safety standards for the first time in almost ten years.

The failure came as 18 of the 26 beaches in West Sussex achieved an "excellent" standard.

A Southern Water spokesman said: "76 per cent of the bathing waters in our region were excellent, with a further 23 per cent of beaches being good.

"In total, 78 out of 79 beaches across Kent, Sussex, Hampshire and the Isle of Wight, met the strict European bathing standard."

Worthing was the only one not to meet the "good" standard and the town will not be able to fly the Seaside Award flag next year.

Of 20 water samples taken from the Heene area only two failed and 16 were of a very high standard.

The cause of the failure is still not known.

An Environment Agency spokesman said: "It's a bit of a mystery. We have gone three years without a failure and then get two samples failing close together.

"The Environment Agency has been conducting intensive sampling to investigate the potential sources."

Haydn Smith, Worthing Borough Council's head of environmental services, said: "There's a possibility of contamination of some surface water and that is being investigated.

"It's an unusual occurrence. The seawater has been of good quality since 1995.

"The test results were quite a surprise. It may have been freak conditions present which we are unaware of."

Deputy leader of the council John Lovell said: "I think it's a one-off. I do not think it will affect visitor numbers to Worthing because it's not a consistent failing."