More than three miles of the River Crouch has been contaminated after 350 gallons of diesel was spilt into the water by would-be thieves.
According to the Environment Agency, no wildlife will be harmed by the spill but nevertheless an investigation into the incident has been launched.
Large floatation devices such as sausages - known as oil booms - line the river to contain the fuel and officers are on the scene to assess the damage at Riverside Walk, Wickford.
It is believed intruders broke into an agricultural warehouse in the early hours of Sunday and attempted to steal the containers - but only succeeded in knocking the contents into the river.
Steve Bewers, team leader of the environmental management team, said: "If we didn't catch it early it could have been a large problem but there has been minimum impact on wildlife in the river."
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