The Maritime and Coastguard Agency and Environment Agency are working in response to a brown liquid being spotted in the sea in Sunderland on Sunday and Monday.
Peter Davison, who was in the Royal Navy for 18 years, took photographs of the substance in the water at Roker and warned people as he walked along the coast.
There have been reports of the spillage also being spotted as far north as Tynemouth and south towards Hartlepool.
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An Environment Agency spokeswoman said: “We are aware that there has been a pollution incident involving a dark brown liquid spillage at sea and that some of this has come ashore in the Roker area. Today (Monday) we received a further complaint of oil pollution in the area and we have sent one of our officers to investigate and collect samples to allow them to be analysed.
“The Maritime and Coast Agency lead on marine pollution and they are undertaking an investigation to find out more details of what happened. This includes using a drone to assess the extent of the pollution and exact locations.
“The Environment Agency will continue to work closely with colleagues in the relevant local authorities, the Maritime and Coast Agency and the Royal National Lifeboat Institution to support the response to the incident.
“If you see a red flag on a beach this is a hazard warning, please take notice for your own safety and make sure you don’t swim in the water.”