Oil washed up along on North Yorkshire beaches has affected up to 50 seabirds, say Yorkshire Wildlife Trust.
The oil began to build up on Sunday afternoon along a 15-mile stretch of coastline.
A clean-up operation is being carried out and beach users have been advised to take care.
Yorkshire Wildlife Trust say the largest reports of oiled birds have come from Scarborough - and warns that this kind of substance can be very dangerous for the animals.
It's not known for sure yet what caused the oil spill.