The National Trust has said it would fight the government to protect the countryside from housing development.
Sir William expressed fears about the future of the green belt
Chairman Sir William Proby claimed some 10,000 acres of the green belt is at risk, which he said was "terrifying".
The countryside was losing its redeeming and restorative qualities "inch by inch", he told the Trust's annual general meeting in London.
The Trust's members are to be asked if it should buy more green belt land to protect it from development.
Prime Minister Gordon Brown has pledged to build three million more homes by 2020 to tackle the housing crisis.
The government says that extra housing is needed to ease shortages, and it can be built while protecting the green belt.
Ministers have said the boom in house building would take place on brownfield sites and areas owned by the public sector.
In his address to the meeting in London, Sir William said plans to build more homes were driven by ministers' "obsession with economic growth".
He said more than three square miles of the green belt have been lost each year as a result of regional development plans.
Sir William said: "What is happening to the countryside? Inch by inch, year by year its redeeming, restorative qualities are being eroded."
He went on: "We are not against progress, nor do we hark back to some mythical golden past. We support the needs of all citizens for decent places to live and we know that some development must happen.
"But the sheer scale of what is being contemplated now goes way beyond this."
Sir William told the meeting that "new arguments" had to be developed to "define the benefits that green belt land and open space bring to us all".
He asked members if The National Trust, which has an annual income of £350m, should buy up green-belt areas to protect them instead of relying on the planning system to safeguard the land.
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Home Information Packs being introduced
All social housing must meet the 'decent homes standard'
100,000 homes in "carbon neutral" communities planned for old industrial sites
New HomeBuy scheme enables social tenants, key workers and other first time buyers to buy share of a home
Sir William's comments come a week after an independent body set up to advise ministers on new homes said that even the government's projected three million figure would not be enough to meet the demand for new homes.
The government said Sir William's analysis was flawed and that he did not fully understand the planning restrictions in place to protect green-belt land.
A spokesman for the Department for Communities and Local Government said the claims made by Sir William were flawed and "misunderstand the planning protections that are in place to safeguard the countryside".
He said: "For the sake of first time buyers and families on council waiting lists, there is a need to build more homes but we believe it is possible to do so whilst protecting the environment and green spaces."
The National Trust has 3.5m members, making it one of Europe's biggest membership organisation.
It owns some 250,000 hectares of countryside and 700 miles of British coastline.