Sixty swans were rescued after hundreds of gallons of oil deluged more than three miles of a town centre river.
The Environment Agency (EA) is now using "absorbent booms" to soak up the oil streak in the River Kennet, in Reading, while they try to track down the culprit.
"Every effort is being made to find the source of this pollution which we believe to have originated in the industrial areas in south Reading," said an EA spokesman.
Around 50 emergency calls were made to the EA when the diesel streak was first spotted in the water, in what is a tributary of the River Thames, in the Berkshire town.
It mirrored a similar incident from 2007 when 60 swans were rescued after a school accidentally dumped cooking oil.
Swan Lifeline began a rescue operation but had to draft in fellow charity Swan Sanctuary, from Shepperton in Surrey, to hook out the royal birds.
"The first unit went out there but couldn't cope," said Swan Sanctuary trustee Steve Knight, whose charity have rescued hundreds of birds across the country because of dumped oil.
"It's a really big problem. If they keep getting oiled they can get oil toxicosis which can lead to death.
"The spill in Reading is certainly a vegetable-based oil, which could be from hydraulics such as brake fluid oil."
The charity, which washed 52 swans down with washing-up liquid and expects to be caring for them for another two weeks, says none of the birds appear to be suffering an toxicosis.
Swans who ingest the oil are given a solution containing charcoal which absorbs the gunk
Wendy Hermon, operations director and founder of rescue charity Swan Support, added: "It's not good on their feathers, they lose their water proofing, but most importantly it is what they are swallowing.
"Sometimes we hear that garages tip their waste in a drain. We have had that in recent situations."
The EA has been known to fine companies as much as £500,000 for oil spills under the Environmental Permitting (England and Wales) Regulations 2010.
Oil firm Essar paid £210,000 to clean up Manchester Ship Canal after an oil spill in 2012.