Discharge from a Thames Water site that killed around 30 fish has been described as “very concerning”.
The water utility said it is investigating a discharge at its Wheatley Sewage Treatment Works site in south Oxfordshire.
A third party is said to have caused the discharge into the River Thame of an undisclosed substance, according to Thames Water and the Environment Agency.
READ MORE: Bicester Motion hangar to be 'sensitively demolished' after fatal fire
Dr Claire Robertson, Oxford rivers project officer at environmental charity Thames21, described the incident as "very upsetting and disappointing".
She said: "This incident emphasises how we are all connected to our rivers - everything we flush down the toilet or put down a drain or sink will enter a river eventually, via surface water drains or sewers.
"We all have a part to play to keep our local rivers healthy: never dispose of chemicals, fuel, paint or hazardous substances down a drain, sink or toilet, take them to a proper disposal site."
Wheatley councillor Tim Bearder described the discharge as “category one, the most serious” and a “chemical incident”.
An Environment Agency spokesman suggested on Monday night that the number of fish killed, which is currently believed to be around 30, could rise.
Liberal Democrat Henley and Thame MP Freddie van Mierlo said he is “extremely concerned” by reports of a “serious pollution incident” affecting the river.
“We don’t yet know the full extent, but it’s clear that something has gone badly wrong,” he said.
READ MORE: JD Vance's visit to Cotswolds will be 'very disruptive'
"I've asked for urgent updates from Thames Water, and I’m grateful for the swift response from local councillors and community members."
He said those responsible must be held accountable.
Ian Middleton, Oxfordshire's Green party leader, added: “It's very concerning, especially as we may not yet know the full extent of the discharge or exactly which chemical was involved and the dead fish may be just the first indication of a more widespread problem.
"As well as the danger to public health, it adds further stress on our waterways that are already under pressure from recent discharges from Thames Water."
He called for a "speedy conclusion" to the investigation and for "appropriate measures" against those responsible.
The Environment Agency's spokesman said: “Specialist officers are investigating a pollution incident at a sewage treatment works in Wheatley, near Oxford, on Sunday.
“We are working with Thames Water to confirm the source and understand what happened.
"A small number of dead fish were seen in Wheatley Ditch, a nearby stream. An official count continues.
“Anyone with information on this or any other suspected pollution in our waterways can contact the Environment Agency’s 24-hour incident hotline: 0800 807060.”
A spokeswoman from Thames Water said: “We have identified the source of the substance as a third-party disposal into the wastewater network, which we have now stopped.
“We are carrying out a full clean of the surrounding sewers and pumping station to ensure they are clean, clear, and free flowing.
“We have deployed tankers, to divert the wastewater flows to an alternative site in order to minimise the impact to the watercourse and we are working closely with the Environment Agency and local stakeholders during this time to ensure full recovery.”
She said the company will pursue an investigation into the source of the substance and vowed “appropriate action” will be taken.
Thames Water did not confirm what was discharged into the river.
Dr Robertson added: "If you are unsure how to dispose of something, use the Oxfordshire County Council 'waste wizard' at https://www.oxfordshire.gov.uk/waste-and-recycling/recycle-repair-reuse."
