Lancashire is the only remaining stronghold of hen harriers in England, conservationists have said.
They fear the survival of the bird of prey in the country "hangs by a thread".
RSPB Figures showed only seven pairs nested successfully this year. Five of those were in the United Utilities' estate in the Forest of Bowland.
According to the annual survey of breeding hen harriers, only 12 pairs even attempted to nest in England.
The number of pairs successfully raising their chicks to fledging was one more than last year, when just six nests were successful.
But it is half the number which nested successfully three years ago.
The RSPB said there was sufficient habitat for 300 pairs of the bird of prey in England.
Dr Mark Avery, RSPB director of conservation, said: "Persecution, associated with land managed for driven-grouse shooting, remains the main reason for the hundreds of missing pairs.
"Even though these birds now have the full protection of the law, the persecution of birds of prey remains devastatingly common.
"Now that the future of the hen harrier in England hangs by a thread, we need to consider all measures necessary to prevent the extinction of this bird in our uplands."
Hen harriers come into conflict with upland grouse shoots as they eat the game birds.
The RSPB is calling on the government to address the problem of illegal persecution, and wants it to confirm that the future of the National Wildlife Crime Unit is secure.
A spokeswoman for the Department for Environment, Food and Rural Affairs said: "Tackling the persecution of birds of prey, such as the hen harrier, is a government priority and we fully support the valuable work of the National Wildlife Crime Unit.
"We are working through our settlement from the Comprehensive Spending Review and will be assessing the potential for the unit's future funding in due course."