A hospital trust spent £4.8 million more than it planned to on temporary agency workers in just four months.
Staff shortages and a higher than expected number of patients meant Western Sussex Hospitals NHS Trust had to pay out more than it intended to make sure shifts were covered.
The agency bill for April to July was almost £5m instead of the £197,000 the trust had budgeted for.
Worthing spent almost £3.5m on agency staff while at St Richard's it was almost £1.5m.
Meanwhile Brighton and Sussex University Hospitals has spent more than £3 million since April and East Sussex Hospitals NHS Trust has spent £1.75 million.
Managers at Worthing say it paid out less in July than it did in June and was working to improve the situation.
St Richard's has been struggling to find permanent workers.
In a report to the trust board, finance director Richard Hathaway said: “From the recent work undertaken regarding temporary staff usage, there still remain local issues.
“Generally, recruitment is proving to be difficult due to a shortage of properly trained staff and the time it takes for an individual to start at the trust due to external checks and references.
“The main areas are in clinical services and woman and children where both are having significant difficulties.”
The trust is continuing to see large numbers of patients coming through both of its accident and emergency departments.
Extra beds have also been opened to cope with demand which means more staff have to be on duty to care for patients.
The trust also has a shortage of junior doctors, particularly in orthopaedics and anaesthetics.
It has already spent more than £500,000 on locum doctors since April and the figure is expected to rise until permanent medics are found.
The trust has drawn up an action plan to cut back on agency staff to not only save money but also provide better continuity of care for patients.
Work will include trying to use more workers from its pool of bank staff and keeping a closer eye on the number of extra beds opened at both hospitals.
A trust spokeswoman said: “We are committed to continually improving the quality of care we offer our patients and ending the use of agency nursing staff is a key element of that.
“Staffing our wards with dedicated Western Sussex Hospitals nurses only will give us the consistency of care that delivers the best outcomes for patients and we are launching a major recruitment drive to achieve this.”