NHS chiefs fear lengthening waiting lists and the looming winter beds crisis could bring Eastbourne's main hospital "to a halt".
Eastbourne District General Hospital is already 385 cases behind its target and the backlog looks likely to increase in the coming months.
The latest statistics also reveal there were 392 patients waiting longer than 12 months compared with 231 at the end of March.
Eastbourne Hospitals NHS Trust, which runs the District General, believes delays in discharging vulnerable patients lie at the root of the problem.
The DGH currently has 31 patients who no longer need to be in hospital but cannot be discharged because social services can't afford to look after them.
With admissions set to rise over the winter months, the board has predicted waiting lists will also rise unless money can be found to free up beds.
Ian Hunt, non-executive director, said: "The situation is immensely serious and it could bring the hospital to a halt."
Chief executive Alan Randall told the board, said: "If we do not address the current problem it will get worse.
"But there's no easy contingency plan we can put into action other than spending money we have not got."
Another factor is the level of staff sickness at the hospital, which is especially high among theatre staff and midwifes.
The board hopes a recruitment drive will ease pressure on hospital staff and cut the amount of sick leave taken.
The shortage of flu vaccine is affecting health services in East Sussex.
Eastbourne Hospitals NHS Trust says it has ordered much more of the flu jab vaccine they it has received.
Hospital chiefs were told the shortfall was due to a Government decision to increase the number of patients eligible for the vaccination.
Monica Green, personnel director at the trust, said: "We have had difficulty getting supplies of flu jabs even though we ordered it nine months in advance."
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