Health chiefs are set to spend six months preparing their case for a new hospital.
Almost 200 props are now in place to hold up the roof at the Queen Elizabeth Hospital at King's Lynn.
A report to managers warns: "There is a direct risk to life and safety of patients, visitors and staff due to the potential of catastrophic failure of the roof structure due to structural deficiencies."
But the QEH, which is operating more than 10 years beyond its originally-intended working life, was not included on the list of 40 hospitals which would be given money for rebuilds or refurbishment, which was announced by the government in October.
Now staff hope it will be one of eight further hospitals which will be given funds for a new build after a government spending review in November.
In a report to Tuesday's meeting of the hospital's ruling board, its chairman Prof Steve Barnett said: "We are entering a critical phase when our aim to provide the kind of sustainable and excellent facilities that our population deserves is being considered at the highest levels."
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A separate report from Chris Benham, the QEH's director of finance, said: "The Trust must have a fit for purpose estate to deliver quality patient care."
Mr Benham adds a new programme board of senior managers has been formed to draw up a detailed case for the new hospital. He says this is expected to be completed by November.
The QEH has been given £20m to pay for repairs to the roof. Some 45pc of the reinforced concrete planks which make up the structure have been surveyed, while 198 props have been installed in 45 different areas of the hospital.
So far health secretary Matt Hancock has declined to answer questions regarding if and when the QEH might be rebuilt. More than 7,000 people have now signed the EDP's petition calling for funding to be made available for a new hospital for King's Lynn.
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