The NHS trust which runs Brighton and Mid Sussex's major hospitals faces a financial crisis after spending almost £3 million it cannot afford.
Brighton and Sussex University Hospitals NHS Trust has already gone more than £2.7 million over budget in the first few months of the financial year.
An emergency financial recovery plan has been drawn up to try to curb spending but hospital bosses insist patient services will not be affected.
The main problem has been the high demand for agency staff to cover vacancies or workers who are off sick or on holiday.
A growing number of patients needing routine operations have also been treated at private hospitals so the trust can meet Government waiting time targets.
Chief executive Stuart Welling said the trust board was concerned about the figures but was confident of bringing the spending under control.
He said: "We are financially struggling but we are working on this. A lot of effort has gone in to looking at the way we approach and deal with agency staff.
"We also spend a lot of money in the private sector and need to significantly reduce that.
"Patients will not be affected. Things are beginning to move in the right direction.
"Obviously the board and I are very concerned and we now have a challenge ahead but we are determined to get it done."
The trust runs the Royal Sussex County Hospital, Sussex Eye Hospital and Royal Alexandra Hospital for Sick Children in Brighton.
It is also responsible for Princess Royal Hospital and Hurstwood Park Neurosciences Centre in Haywards Heath and provides some services at Brighton General Hospital.
It has an annual budget of about £250 million.
The trust is not the only one struggling financially in Sussex.
East Sussex County Healthcare NHS Trust, which provides mental health, learning disability and substance misuse services for thousands of people across the county, is also facing critical problems.
It is already almost £1.7 million over budget and director of finance and support services Chris Fincham has warned that without urgent action it could end the year with an "unacceptable" £4.7 million overspend.
In a report to the board, which meets today, Mr Fincham says trust's serious financial position means making savings and managing known problem areas such as high drug costs.
Staff recruitment problems need to be given high priority.
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