The National Trust plans to cut 250 jobs from offices across the country in a "shocking" move, a union has said.
The National Trust owns the area of Stonehenge
In a statement the union, Prospect, blamed the move on "needless penny pinching" and denounced the proposed cuts as a "kneejerk reaction".
The National Trust is the country's largest non-government landowner and employs almost 4,000 full-time staff.
It said savings were needed to meet rising costs and hoped the cuts would be made by voluntary redundancy.
The Trust, a registered charity, owns more than 248,000 hectares (612,000 acres) of British countryside and almost 600 miles of coastline.
It also runs 200 historical houses and 49 monuments and mills.
Prospect said offices in London, Swindon, Cirencester, Devon and Cornwall, East England, East Midlands, Northern Ireland, the North West, South East, Wales, West Midlands, Yorkshire and the North East were at risk of redundancies.
It said the Trust was facing a short-term financial shortfall after conducting a review of its internal organisation, buying new information technology and funding its major projects.
The union's negotiator, Helen Stevens, said: "This is a shocking decision. It is a major blow to the National Trust and will wipe out around 5% of its total workforce.
"Losses of this scale will make it almost impossible to avoid compulsory redundancies.
The Trust says all profits go back into the company
"We have yet to see any evidence that these cuts are needed or that it is more than a knee-jerk reaction by the Trust's over-cautious trustees."
She said the Trust could have made the necessary savings without culling jobs if it had waited for efficiency savings to start feeding through.
She added: "It makes no sense to lose the lifeblood of the Trust. This is more like selling the family than the silver."
A Trust spokesman said the organisation was facing rising costs, increased National Insurance contributions and increased pension payments. It also wanted to increase its operational fund to £20 million. About 50 jobs have already been earmarked through voluntary redundancy and natural wastage, the spokesman added, insisting: "This is about safeguarding our conservation for the future."