A pair of swans have been found drenched in diesel close to the Aire and Calder Canal.
The birds were rescued by volunteers on the waterway off Fox Holes Lane, Altofts, on Monday.
Rescuers found the two swans had been contaminated by the fuel and they are now receiving treatment at the wildlife charity Yorkshire Swan Rescue.
Dan Sidley, founder of Yorkshire Swan Rescue, said: “It is vitally important the birds are rescued and thoroughly cleaned to remove the diesel contamination.
“Pollution of this kind strips the waterproofing from the feathers of the birds, and they will either drown or die from hypothermia in freezing weather conditions.
“This is one of the worst cases involving diesel pollution that I have seen in recent years, and the swans will have to stay here at the sanctuary for a long time before they are fully recovered.”
The charity will care for the birds at its nature reserve in Barlow, near Selby, until they are able to return to the canal.
The incident has been reported to the Environment Agency and officers will now investigate the cause of the pollution.
Mr Sidley urged members of the public who come across any animals covered in diesel to call 07763424892.