A destroyed sewage tank that spilled up to 900,000 litres of storm sewage is the centre of an investigation into pollution levels in the River Ouse.

The tank at Ham Lane sewage works in Lewes collapsed during fierce storms last December, spilling its contents onto the site and threatening the river.

A spokesman for the Environment Agency said: “We are still investigating the incident which resulted in the storm tank collapsing. The results of our investigation will determine any enforcement approach that we decide to take.”

The collapsed tank was one of the site’s emergency storage units, which are in place for when rainfall overwhelms nearby drainage systems.

The Environment Agency is looking to determine why the tank collapsed and the effects that the spilt pollution may have on the Ouse.

Nine temporary tanks are now in place on the Ham Lane site and these tanks collectively have a higher storage capacity than the original unit.

A spokesman for Southern Water said: “Following the recent high volumes of rainfall and resulting high groundwater levels, this temporary situation needs to continue in the short term until the water levels subside and flows can be diverted to the treatment works in Newhaven.”

The Environment Agency is also questioning how many times Southern Water has discharged contaminated water into the River Ouse over the past two years.

Southern Water was fined £200,000 last year for discharging raw sewage on to beaches at Thanet, Kent, in 2011.

The Ouse runs from Lower Beeding, West Sussex, to its mouth in Newhaven in East Sussex and has a rich ecology.

It is known for its large variety of fish such as sea trout, carp and pike, and is a popular site for anglers.

A spokesman from the Ouse Angling Preservation Society said: “We deal with all matters of waste or pollution absolutely seriously and we appreciate the speedy response of the Environment Agency.

“Pollution can affect fish stocks and migratory fish, but we have not seen any effects as of yet.”

The Environment Agency has said that due to high levels of rainfall over the winter, the spilled water is likely to have been very diluted by the time it entered the watercourse.