Worthing Priority Care NHS Trust is facing a financial crisis after overspending its budget for the first half of the financial year by £253,000.
The figure is set to rise to well over half a million pounds by the end of March.
The problem arose because the trust was unable to recruit permanent key psychiatric medical and nursing staff and had to use expensive agency workers instead.
The overspend, the first in the trust's history, came despite a 25 per cent budget increase compared to the same time last year.
The trust overspent on medical staff by £38,000 in April. The figure had reached £68,000 by September.
A financial review carried out by the trust at the end of September predicted it would overspend by £547,000 over the whole year even if non-essential spending was cut.
The recruitment difficulties have had a knock-on effect in other areas of the trust.
For example, not all the 48 beds at the Meadowfield unit for acute mental health patients, opened in October, can be used. Instead the trust is having to spend £76,000 on private beds.
Brian Holmes, director of service development, said: "We are having trouble recruiting and retaining medical and nursing staff due to national shortages. The nature of the work means there is a high turnover, especially with psychiatric staff. They are a scarce resource.
"We have to keep residential staffing levels up and that is why we have had to go to expensive agencies. That is why we have such a big overspend.
"We are in a dilemma as we have to maintain staff levels and provide a service to local people."
Mr Holmes said that in a bid to solve the problem the trust had stepped up its recruitment drive, with some success.
He said: "We have signed new staff up but many have long notice periods to work out, so we will not see an immediate difference. That is why we have predicted a £500,000 overspend by the end of the year.
"Once an overspend has occurred, it is difficult to get money back unless the demand for the service decreases which is not likely. But we are doing everything we can to balance the books.
"There will be no cuts in services and any cuts will be on non-essential expenditure such as supplies."
The trust runs the Meadowfield unit for acute mental health patients and two psychiatric units at Southlands Hospital, The Ridings and the Birchfield wards.
There will be a shake-up of mental health facilities at Southlands with the closure of the Ridings and Birchfield wards in March. Elderly and mentally ill patients will be transferred to a new site in Worthing.
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