Almost 12,000 grey squirrels have been killed in Northumberland in an effort to save the red squirrel.
Greys are a major threat to the UK's native red squirrel
New figures reveal that two-thirds of the county's grey population, which spread a lethal pox virus, have been culled with government backing.
UK native red squirrels are dying out because they cannot compete against the larger grey populations.
The RSPCA said it "questioned" long-term culling and hoped the animals had been killed humanely.
The Red Squirrel Protection Partnership (RSPP), funded by environment department Defra, carried out the cull and targeted grey populations from the Scottish Borders to the River Tyne.
In just over a year, 11,615 have been culled using "humane pest control", according to the group's chairman, Liberal Democrat peer Lord Redesdale.
There are two methods of culling - poisoning with warfarin or shooting the animals after being lured into traps with hazel nuts.
Lord Redesdale said: "Once we're tipped off about the greys, we move in and set traps which are instantly effective.
"We can clear an area in matter of days, using staff specially trained in humane methods.
"Some people believe it is impossible to control the spread of grey squirrels, but we have proved it can be done, and reasonably cheaply."
Greys were introduced from North America in the 19th Century and have led to a reduction in reds because of their higher breeding rates.
There are estimated to be only 140,000 red squirrels left in Britain, with over 2.5m greys.
According to the RSPP, red and grey squirrels cannot live alongside each other, because reds cannot compete for food against the larger and more powerful greys.
Since 1930, it has been illegal in the UK to release a grey squirrel after it has been trapped, and it must be humanely killed.
The reds have a strong-hold in Kielder Forest, but the RSPP predicts they will be extinct in Northumberland within 10 years.
Grey squirrels are also considered a threat to woodland because they strip bark from trees.
A spokeswoman for the RSPCA said: "We question whether culling is a long-term solution to this problem.
"But if the scheme is in coalition with Defra we would hope and expect that animal welfare is a priority and everything is being done under licence and is within the law."
The RSPP, which received £148,000 funding, is alerted to grey colonies by landowners or members of the public.
The RSPP now plans to move on into County Durham.