Wards may have to be re-organised at two hospitals to reduce over-spending by almost £5m.
The hospital trust has spent £11.2m on temporary staff
Managers at the Royal Sussex County Hospital in Brighton and Princess Royal Hospital in Haywards Heath have revealed the budget crisis six months into the financial year.
They have warned they will have to re-open closed wards and cut their use of private hospitals to try to balance the £4.7m over-spend.
A spokesman for the Brighton and Sussex University Hospitals NHS Trust said the action taken would not have a negative impact on patients.
The trust has blamed its financial situation on the high cost of agency and locum staff, with £11.2m spent in the first six months of this year.
A spokeswoman said: "This is mainly due to difficulties in the South East with recruiting staff - we currently have 200 vacancies for nurses, for instance.
"The main factor is the high cost of living."
She said the trust had also been forced to pay £1.6m for patients to be treated in the private sector because it did not have enough beds, operating theatres and staff.
So-called bed blocking, where patients are fit to leave hospital but have no provision for their care when they return home, has exacerbated the problem.
David Dumigan, director of finance for the trust, said: "We are struggling to compete with the agencies, who are offering higher rates of pay."
He said the trust had increased its hourly rate for bank, or temporary, staff in an attempt to lure them away from the agencies.
Mark Sergeant, of Unison, said: "We have got to recruit staff, otherwise it is paying the agencies and basically putting the money into the private sector.
"A lot of that is not going into health care then, it is going into shareholders' profits for a company rather than health care.
"We would much rather pay that money to retain staff and recruit staff and then the benefit is actually seen by the NHS."
The trust hopes by attracting new staff it may be able to cut its rate of over-spending to £1m by the end of the financial year.