Experts remain baffled about a chemical which dyed a popular outdoor swimming pool.

Hinksey Pool, in Hinksey Park, South Oxford, has been closed since Saturday morning after vandals put a chemical into the water on Friday night.

It was strong enough to dye the entire 350,000-gallon pool a garish purple colour. Disinfectant and filtration has brought the 55m by 35m lido back to a more natural colour.

Oxford City Council was hoping to get the results of a microbiological test today.

It had been feared the pool could be closed for up to three weeks. However, there are now hopes it may reopen next Monday.

If the water is safe, it will be drained into the sewerage system. It would then take another four days to be refilled, reheated and reopened. Oxford City Council is thought to be losing about £2,000 each day.

The colour has mystified local swimming pool experts.

Eddie Bjarnson, managing director of Aqua Services, in Weston on the Green, said: "I have no idea what they could have done in a pool of that size.

"The only chemical I know would be something like iodine, which is a very strong purple - but how would you get hold of iodine unless you were a chemist?"

Barry Cavey, director of Yarnton-based Blue Water Pools, said: "It's very strange. There's nothing obvious that would cause the pool to go like that."

David Rundle, deputy leader of Oxford City Council, said up to 1,000 people would visit the pool on a hot day.

He said: "Quite a few people who haven't been there do not realise the seriousness in terms of loss of a service and a loss of revenue to the council. I'm hoping the culprits will be big enough to stand up and say they have done it."

Mr Rundle said the council would review security measures, as it was the second attack by vandals in less than a fortnight.

On July 28, vandals tried to damage the controls of the Splash facility next to the pool.

A Thames Valley Police spokesman said officers were continuing to investigate.

Opinion remains divided on oxfordmail.net as to whether the dyeing was a harmless joke or something more serious.

One anonymous post said: "Good to see old fashioned pranks are still about!"

'Lizzie' replied: "Its a shame a few a mindless people ruin something for all."