A bird rescuer claims Crawley Borough Council has not done enough to protect baby swans on Ifield Mill Pond.
John Potts, of Swans and Friends Bird Rescue, said he warned Crawley Borough Council that a brood of cygnets was at risk of being washed down the weir.
A week later he said at least four out of six cygnets had been separated from their parents.
Mr Potts rescued one, which is now at the Redhill sanctuary, but the other three are feared dead. Adult swans reject their offspring if separated for more than 24 hours .
Mr Potts said a log across the mouth of the weir was needed to stop the brood being washed downriver.
He said: “It’s very sad because the chances are they will just perish, foxes will take them. The parents will refuse to accept them and beat them to death.
“We just need the council to do something quickly. I’ve left a message for them but they’ve done nothing.
“By next week you will probably find there will be no cygnets left.”
A council spokesman said: “It’s not possible to put a cygnet-proof barrier across the spillway because it would contravene reservoir legislation and the potential safety of the structure and property downstream.
“Improvement works at Ifield Mill Pond are planned to begin next year and, if viable, we will endeavour to incorporate additional measures as part of the proposed scheme.”
Cllr Chris Oxlade (Lab, Ifield) said: “It’s something we’ve been working on for a number of years.
“We’re not going to take no for an answer. There should be a solution to protecting wildlife without swans getting washed away.”