The damage caused by diesel leaking into the River Test could have a devastating impact for years to come, according to the Hampshire and Isle of Wight Wildlife Trust.
The Trust's Lower Test nature reserve is experiencing high levels of fuel oil pollution, which could significantly impact the health and integrity of the protected habitats within it.
It says the death of a cygnet and fish have been linked to the pollution, which is believed to be caused by an oil interceptor overflowing in heavy rains.
John Durnell, Hampshire and Isle of Wight Wildlife Trust
Over the past few months the wildlife trust says it has contacted Southern Water asking them to identify the source.
Southern Water says it is working with the Environment Agency to minimise the risk of future pollution incidents.
WATCH: ITV News Meridian's Rachel Hepworth reports on pollution in the River Test.