The female from England's only pair of nesting golden eagles is feared dead.
Golden eagles have nested in Cumbria since the 1950s
Her absence from the breeding valley in the Lake District has prompted concerns she may have died, according to experts from the Royal Society for the Protection of Birds, (RSPB).
She was the second female eagle to occupy a remote valley site at Haweswater, near Penrith, and first arrived in the Lakes in 1981.
It is believed that she would have been at least 28 years old this year.
The bird would normally have been sighted at this time of the year
RSPB wardens will continue to search nearby valleys in the hope that she may be located, but are now becoming increasingly convinced that she has died.
Golden eagles have been present in Cumbria's Lake District since the late 1950s and first nested in the Haweswater area in 1969.
However, the two eagles that have most recently occupied the area are not the original birds, and both the male and female of the pair have changed several times over the years.
The RSPB says the male golden eagle is still in the Haweswater area and is putting on spectacular display flights, probably in the hope of attracting a new female to join him.
The male is the third male eagle to take possession of the Haweswater territory, and arrived in 2001.
Bill Kenmir, RSPB Haweswater reserve warden, said: "This is sad news, the female eagle was a great favourite with local people and regular watchers, and over a period of more than 20 years had successfully reared nine young.
No eggs produced
"The sight of her soaring flights over the valley will be missed by all our wardens, reserve volunteers and visitors."
The current male bird is believed to be a youngster in eagle years and is probably only seven years old.
The previous male disappeared in late 2001 when he was at least 30 years old - at the time, he was the oldest known British eagle.
Since 1969, the various Haweswater golden eagles have produced 16 young, but none have fledged since 1996 and no eggs have been laid in four of the last five years.
The 9,500 hectare Haweswater Estate is owned and managed by United Utilities, including a partnership with The RSPB.
The UK population of golden eagles is around 420 pairs, and the nearest birds to Cumbria are in southwest Scotland.