The union represents more than 1,000 members in the Trust.
Prospect 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.
"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. The impact of this decision will be felt by the staff who remain almost as much as by those being made redundant."
250 full-time posts are being cut by the Trust at 14 regional offices to achieve savings of £6m. The offices affected are at: Central offices in London, Swindon and Cirencester; Devon and Cornwall; East of England; East Midlands; Northern Ireland, North West; South East; Thames and Solent; Wales; Wessex; West Midlands; and Yorkshire and North East.
The Trust says it is facing a short-term financial drain on operating capital created by two major projects, an organisational review and new information technology. These capital projects were designed to improve efficiencies over time, after an initial short-term outlay.
Given time, the books would balance, said Stevens. "But rather than wait for the efficiency savings resulting from the two projects the trustees have decided to boost general operating capital by £20 million by 2006-07 and build up reserves."
The cuts fall predominately within the Trust’s 11 regions but rumours suggest that staff based at National Trust properties may also be affected.
Stevens added: "The Trust is as much about its people as it is about buildings. It makes no sense to lose the lifeblood of the Trust. This is more like selling the family than the silver – who will deliver on the future of the Trust when so many staff have gone and those who are left feel demoralised and over-worked?"
Prospect has called on the Trust to work with the union to achieve savings and stop waste; improve its redundancy terms; widen the redundancy pool; improve severance terms for the over-55s; cut back on agency staff; redistribute the remaining workload; and institute a fair selection process for redundancy.