Businesses and householders in Worcester have been told to check their oil tanks to make sure their fuels are stored safely.
The call from the Environment Agency comes as there have been a number of incidents where oil has polluted the Worcester and Birmingham Canal.
It has impacted a two-mile stretch of the waterway and seven swans required treatment, the agency added.
It believes diesel had been entering the canal via one of its tributaries, many of which have been covered over time so are not easily visible.
The Environment Agency said a number of Household Recycling Centres accept small quantities of oil from members of the public while businesses must ensure their waste oil is collected by a registered waste carrier.
Dave Throup from the Environment Agency added : “We are working with the Canal and River Trust to stop diesel and other oils entering the canal and to trace the source of this oil. We are asking businesses and householders in Worcestershire to check any oil tanks or underground oil pipes on their property to make sure they aren’t leaking and to make people aware never to pour oil or fuel down the drain.”