RSPB Scotland said the bird of prey's population in Scotland is in trouble, with a 20% decline in numbers over a six-year period.
It is calling on grouse moor managers and gamekeepers to do more to prevent their "illegal persecution".
The harrier is a natural predator of the red grouse but the conservation charity said techniques such as providing alternative food for them have proven effective and should be more widely embraced.
However, gamekeepers said the charity needs to assess the "bigger picture" of hen harrier decline and the drop in numbers of smaller birds of prey.
The Partnership for Action Against Wildlife Crime in Scotland (PAWS), which includes the police, land managers, conservationists and the Scottish Government, launched a project last year aimed at raising awareness of the bird's plight.
RSPB Scotland said work has also been carried out at the Langholm Moor Demonstration Project over the last seven years to reconcile "sustainable grouse shooting" with maintaining a "viable population" of hen harriers.
Duncan Orr-Ewing, the charity's head of species and land management, said: "This work has clearly shown that using techniques such as diversionary feeding of harriers during the breeding season markedly reduces their predation of red grouse.
"It is exceedingly disappointing that so few sporting estates have used this legal and effective management tool. The illegal killing of hen harriers is condemned by all of the PAWS partners, and needlessly threatens the population of one of our most spectacular and rarest birds when there is a practical solution."
A spokesman for the Scottish Gamekeepers Association (SGA), said: "In terms of accusing grouse moors alone for the decline of hen harrier, the RSPB, as a bird charity, could spend donor money more wisely by assessing the bigger picture of harrier decline and the criminal drop in the smaller, less iconic prey birds, rather than spending it on demonising gamekeepers; the vast majority of whom work within the law every day, under very trying circumstances, to produce a balance of species as well as a surplus of grouse to shoot."
He added: "Diversionary feeding is supported by the SGA, which advocates legal tools for species conflicts, but faith in its deployment will depend on it being proven to work when harrier numbers rise. As yet, there is no evidence to show this is the case."