The fight to conserve one of the UK's rarest birds has been thwarted by torrential rain wiping out nests during the breeding season.
In 1997 a survey revealed only 11 bitterns across the UK
The RSPB had a programme to boost numbers of bitterns on nature reserves across the Suffolk coast but has seen nests washed away in flash flooding.
It hoped to increase the numbers of bittern, a rare type of heron. There are only 44 breeding males in the UK.
Loss of chicks will have a knock-on effect across the UK, the RSPB said.
Bad weather has led to the loss of all but two of the nests, an RSPB survey has shown.
'Glimmer of hope'
There had been nine nests, including five at Minsmere reserve and one at the North Warren reserve.
Adam Rowlands, site manger for Minsmere and North Warren, said: "This is a serious set back for the bittern.
"Suffolk has been the engine room of the species recent recovery in the UK.
"The one glimmer of hope is that the birds did get off to an early start this breeding season and there may still be time for some of them to try again, but it is getting late in the year.
The male bird makes a booming sound to declare a territory and attract mates, the distinctive sound can be heard up to 3 miles (5km) away.
In 1997 a survey found only 11 booming males across the UK, but numbers had grown to a 50-year high of 55 in 2004.
Dr Mark Avery, the RSPB's director of conservation, said: "Despite our success in recent years, bittern numbers remain perilously low.
"That one spell of bad weather can have such potentially serious consequences is proof of that."