Hospital bosses say they will continue to struggle to meet Government targets unless more is done to tackle bed-blocking.
More than 100 patients are stuck in hospital beds in Haywards Heath and Brighton, waiting to be discharged.
The longer they stay in hospital, the greater the impact on accident and emergency waits and the higher the number of cancelled operations because no beds are available.
This leads to more financial pressures with patients being referred to private hospitals for treatment for non-urgent operations so waiting list numbers can be cut.
Brighton and Sussex University Hospitals NHS Trust chief executive Stuart Welling told board members yesterday it was vital to get the number of bed-blocking patients down.
He said: "At the moment there are about 118 delayed discharge patients waiting to leave hospital. This is not good practice and not good health care.
"If it is not dealt with we will not have the capacity to do the work we need to achieve targets."
The trust is using reserves and some capital funding to clear an overspend of more than £5 million for the last financial year.
Financial director David Dumigan said a long-term financial recovery plan included reducing the number of patients referred for private treatment.
He said: "We need to get people out of private beds and back into our own NHS beds.
"The problem with delayed discharges is not just something the trust and the primary care trust need to tackle but we also need to work closely with social services."
Using the private sector has cost the trust between £5 and £6 million, including £3 million at the Princess Royal Hospital in Haywards Heath.
More than £600,000 is being spent on increasing nursing levels at the Princess Royal.
About 40 beds that have never been used are also being opened to increase availability.
Bed-blocking patients are mainly elderly, waiting for a nursing or care home place or arrangements to support them in their own homes.
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