The spill happened at Swan Haven Park in Crestwood Road, Moulton, last weekend and the Environment Agency put out barriers to contain the diesel.

However, the barriers were removed from the water on Tuesday and the oil spread, affecting the swans.

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Fortunately, the two adult swans and their three cygnets were caught before the diesel caused any skin burns and the swan family were taken to the RSPCA East Winch Wildlife Centre in Norfolk to be washed. They should make a full recovery.

RSPCA inspector Cherry Evans said: "The swans should now be fine and we haven't seen any more affected birds in the area, although the pond is popular with coots, moorhens and ducks.

"We have been told that local youths took the oil barriers out of the water and allowed it to spread.

"If this is the case, the RSPCA totally condemns their actions as it means that more wildlife could be affected by the spill.

"We hope to go back to the area and clear some of the litter from the pond as people have thrown all sorts of debris into the water which could also pose a threat to more wild birds."