Seagulls roosting under a seafront pier could be responsible for a drop in water quality which could lead to the local authority warning people not to go into the water at one of its most popular bathing beaches, a water company has said.

Portsmouth City Council is investigating what is causing the fall in quality at the sampling point, west of South Parade Pier on Southsea seafront.

Southern Water is working with the authority and the Environment Agency and has suggested the issue could be caused by gulls gathering under the pier or possibly by leaks from small pipes as there is no sewage outfall in the area.

By looking at the samples we can find out the type of animal source it comes from

A spokesman for the firm said: “Seagulls are roosting under the pier which can be a problem.

“We have to speciate the E.coli; by looking at the samples we can find out the type of animal source it comes from.”

Steve Pitt, leader of the council, said that if the water at Southsea East was rated by the Environment Agency as poor later this summer it would be forced to advise people not to go into the sea on that stretch of beach from next May.

He said: “This is an issue where all organisations involved are working together as urgently as possible to find out what’s going on and fix the problem.”

The investigation has examined possible sources including ongoing coastal defence works, leaks from the pier, contaminated groundwater, as well as potential sewage releases by Southern Water, but the council says there are no sewage outfalls in the area.

A spokesman said: “Main sewerage pipes seem to be working properly, so the main focus is now on smaller pipes connecting to them.”

We are continuing to support our partners in finding solutions, and are actively carrying out water quality sampling tests to help direct further investigations and action

A Southern Water spokesman said: “We take any decline to water quality in our region extremely seriously, and have been working closely with the Environment Agency and Portsmouth City Council for more than a decade to tackle concerns in this area.

“Over this time, leaking ageing sewers have been identified as one contributing factor, prompting a major upgrade programme involving sealing pipes with special polymer linings and ensuring manholes are watertight.

“Others contributing factors include surface water run-off from roads and pavements, wildlife such as seagulls roosting beneath the pier, and private sewer pipes wrongly connected to surface drains.

“We are continuing to support our partners in finding solutions, and are actively carrying out water quality sampling tests to help direct further investigations and action.”

An Environment Agency spokeswoman said: “All agencies involved are working to identify, remove and reduce sources of pollution to ensure that the status of this bathing water improves. The pollution sources appear to be localised and possibly coming from contaminated groundwater.”

Southern Water is facing a private prosecution in relation to unrelated water pollution in the River Test from a surface drain at the Nursling Industrial Estate near Southampton.

Not-for-profit group Fish Legal has issued summons against the company with the first hearing due at Southampton Magistrates’ Court on September 28.

A spokesman said: “The world-renowned River Test is a rare chalkstream habitat, one of only around 200 such rivers in the world.

“It is a Site of Special Scientific Interest supporting Atlantic salmon, otters, water voles, brook lamprey and bullhead, but less than 18% of it is in ‘favourable’ condition.

“The section between Romsey and the estuary, the focus of the criminal case, is currently classified as ‘unfavourable’ due to polluting discharges.”

George Graham, chairman of Fish Legal, said: “Both the water company and regulator are fully aware of the persistent pollution coming from this outfall. We cannot stand by any longer and wait for them to act.”

A spokesman for Southern Water said: “In June 2021, we were made aware of a release of diesel in the area of the Nursling Industrial Estate that escaped into the Little River Test.

“Our teams worked quickly, and with other agencies, to protect the watercourse and surrounding environment. An Environment Agency investigation into the cause of the incident and the identity of the polluters is ongoing. ”

The Environment Agency spokeswoman said: “We take every report of pollution seriously and if permit breaches are found we will take enforcement action, up to and including prosecution.

“Our officers have been on site to undertake testing of the River Test and will be interviewing Southern Water at a later date to inform our next steps. As we are currently continuing our investigations, we are unable to comment further at this time in order not to prejudice our inquiries.”