About 140,000 homes lost their mains supplies when the plant was overwhelmed by flood water last month.

Customer relations director Martin Kane said its flood defences were considered strong enough to withstand heavy rain.

"The site was built above the flood plain, so the protection was considered to be appropriate," he said.

Steep learning curve

Risk assessment of the Mythe treatment works in 2004 put the chances of the flood defences breaching at 1 in 1000, he said.

A 6ft (1.82m) flood defence wall has now been built around the works.

Mains water supplies have now been restored to the homes affected, but the water is still not safe to drink.

Mr Kane said chemical and bacterial tests were still being carried out.

Drinking water is still available from special distribution points and bowsers around the county.

The water company is one of several organisations which have been examining their responses to the flood crisis.

Chief Constable Dr Tim Brain, of Gloucestershire Police, said it had been a steep learning curve for all involved.