SCIENTISTS have said the purple water colouring at Oxford's open-air Hinksey Pool was caused by a chemical called potassium permanganate.

Last night, Oxford City Council leisure chiefs got final confirmation of the cause of the discolouration and the all-clear from health and safety inspectors to re-open the popular Hinksey Park facility tomorrow morning.

It would now appear someone poured the chemical compound, which turns green and then violet when mixed with water.

Potassium permanganate is an easily-available chemical, which has many applications. These include use as a disinfectant for dermatitis, infected cuts and fungal infections of the hands and feet.

Police were called to the pool on the night of Friday, August 10 after reports of a disturbance. But officers could not see the water had been discoloured because it was too dark.

It was only the following day - one of the hottest of the year - that the full extent of the 'pollution' became clear.

Oxford city council deputy leader David Rundle, who is also responsible for leisure, said: "The health protectorate has given the all clear.

"My understanding is that traces of potassium permanganate were found, but that the levels were safe.

"Why isn't the pool open today? Well, we are back-washing the filters to make sure there is absolutely nothing left in there.

"Some people may like to call it high jinks, but it has caused serious disruption - in particular on Saturday, August 11, which was one of hottest days of the year.

"Since then we have had poor weather and that has meant less people have been inconvenienced.

"What I am hoping is that since the pool closed when it was warm weather, the better weather will return when it re-opens."

City council leisure chiefs have promised a full and thorough review of security at the open-air pool, which is the only one in the city, to ensure a similar prank never happens again.

Last night Steve Holt, city council leisure operations manager, visited Hinksey Pool to ensure all was in order before tomorrow's re-opening.

Police investigating the incident are yet to arrest anyone in connection with the crime. They are keen to talk to anyone who was in the area on the evening it happened who may have witnessed any suspicious behaviour.

On a hot day, the pool nets the Town Hall about £2,000 - with up to a thousand people coming for a splash.

It costs £4.40 for an adult swim at the pool - or £3.40 for early and late sessions - while children can swim for free.

What has worried the city council is the vandalism was the latest in a spate of attacks on leisure facilities in the area, with the Splash facility next to the pool damaged last month.

Anyone with information is urged to call Oxford police on 08458 505505.