Eighty swans have been removed from the River Thames due to a mystery oil spill which is baffling authorities.
Thames Water was called out at 10:30pm last night to the stretch of river between Windsor Bridge and Eton Bridge.
Bob Lang, a volunteer at the charity Swan Lifeline, said he had seen scenarios of this nature in his 20 years at the charity but "nothing like this".
The Environment Agency, with assistance from Thames Water, is investigating the source of the spillage but said it is "quite unlikely" they will find the source due to most of the oil having dispersed leaving just isolated pockets.
Mr Lang said: "I've seen it before but nothing like this. I've been coming for about 20 years."
Around 20 volunteers have been working to remove the swans from the river and so far about 80 of them have been lifted off - with more work to do, Mr Lang said.
"We have a set procedure. We have to get the oil off the feathers and then we dry them and let them preen themselves as much as they can.
"They have to be with us for a few days," he said.
Mr Lang said that swallowing oil can damage the swans' internal organs.
He added: "Whoever did it appears to have got away with it."
A spokeswoman for the Environment Agency said a member of the public contacted them to report a "pollution in the river".
The Environment Agency then contacted Thames Water due to the possibility it was coming from the surplus drainage network.
The spokeswoman said: "There's no guarantee we'll find the source."
An oil-covered swan compared with a normal swan
In a statement, Thames Water said: "We are currently assisting the Environment Agency to try and determine the cause of an oil spill in Windsor and to help with the clean-up operation.
"Our crews have been out since 10.30pm last night but it is still not clear how this has happened."
In a statement the Environment Agency said: "The Environment Agency received reports last night from members of the public that oil was seen on the River Thames near Eton Bridge in Windsor.
"We immediately contacted Thames Water who attended the site overnight to help identify and isolate any potential source of the pollution.
"Environment Agency Officers have been on site working with Thames Water contractors. They found that oil entering the River Thames had stopped with no oil coming from any of the surface outfalls or other potential sources.
"Most of the oil has dispersed and it is therefore impractical to deploy booms to trap the remaining oil.
"High dilution of the River Thames has minimised the impacts of the pollution.
"In order to minimise the possibility of further oil entering the Thames we will be carrying out riverside checks and will continue to monitor the situation."