The UK's smallest and rarest swan has suffered an "alarming crash in numbers", the Wildfowl and Wetlands Trust has said.
The Slimbridge-based charity said more than a third of Bewick's swans have disappeared since 1995, when the total population peaked at 29,000.
The latest figures show that, by 2010, there were just 18,000 left.
Scientists believe illegal hunting, power lines and lead poisoning have contributed to the drop in numbers.
The charity said it feared the next census, due this winter, would reveal a "further, more worrying decline" in population.
In some winters, the Ouse Washes spanning Norfolk and Cambridgeshire receives 33% of the northwest European Bewick's swan population.
Head of UK waterbird conservation Eileen Rees said swans were not producing enough offspring to replace the ones that have died over the year.