More than 60 of the animals were taken to a wildlife hospital after they were found coated in oil at Brayford Pool marina on New Year's Eve.

They were treated at Norfolk Wildlife Hospital in East Winch.

RSPCA staff have spent up to two hours cleaning each bird and the first 12 swans are to be returned to the water on Friday.

It will be another fortnight before all the birds are returned.

Environment Agency staff are continuing to investigate the source of the oil leak.

It was initially thought the oil came from a boat which sank following a rise in water levels caused by heavy rain.