Rare freshwater pearl mussels have been bred for the first time in England, at a secret location in Cumbria.
The mussels will be returned to the River Lune
The Environment Agency says more than 2,500 pearl mussels have been bred as part of a major conservation programme.
Most of the mussels, which produce valuable pearls, are to be reintroduced to the River Lune in Lancashire.
The Lune mussels have been unable to successfully breed in the wild for more than 30 years, putting the colony at risk, with only 80 adults left.
The mussels can live to more than 100 years old, but changes in river water quality and reduced numbers of salmon and trout hosts for larvae, have seen numbers plummet.
The largest and most viable population remaining in England occurs in Cumbria.
Nicola Barnfather, the Environment Agency's biodiversity officer, said: "All pearl mussel populations in England and Wales are struggling in their natural environment as the conditions in the rivers are no longer suitable for young mussels, which need pristine water quality.
"This is part of a wider international problem as we've lost over 90% of the European population in the last 100 years.
"If we had not intervened and removed the mussels for a captive breeding programme they would have very soon become extinct in Lancashire.
"We hope to rear the juveniles until they are about five years old. At this point we can return them to a suitable part of the River Lune so the population can once again become self-sustaining."
The mollusc is one of the oldest river species in the UK, and freshwater pearls can be seen on many portraits of Queen Elizabeth I, who reigned from 1558 to 1603.
Experts in Wales managed to breed several thousand of the mussels in 2006.
But none have been bred in captivity in England until now, according to the Environment Agency.