The bill for providing agency cover at the three biggest councils in Sussex has risen to £22 million in the last financial year.
For Brighton and Hove this figure has more than doubled – jumping from £3.4 million in 2011/12 to £7.1 million in the last financial year.
Unions said some agency staff had remained in their “temporary” posts for more than two years and were no longer “cost-effective”.
The rising bill has been partly blamed on staff sickness but also includes temporary staff covering short-term and maternity contracts.
West Sussex County Council has also seen a rise in agency costs during the past three financial years – increasing by 10% from 2011/12 to £8.4 million in 2013/14.
In some of the most under-pressure departments, staff are missing the equivalent of one in every 22 work days through sickness.
Staff working in the council’s adult services, children’s services, fire and rescue service and communities commissioning all missed on average at least 3.5% of work days in 2013/14.
East Sussex County Council saw its agency bill drop in the last financial year by £400,000, from £7 million in 2012/13.
The council lost 37,676 full working days through sickness in 2013/14 – an average of nine working days missed by staff.
Mark Turner, GMB branch secretary, said management was turning to agency staff in part because it was easier to “let workers go”.
He said one of the added agency uses may have come out of the recent Cityclean pay dispute and provided cover needed for workers refusing to work extra Saturdays.
He said: “Agency costs have been a constant bugbear of unions and we have constantly questioned the council on its usage of agency staff.”
Leatham Green, East Sussex County Council assistant director for personnel and training, said a large proportion of the working days lost were accounted for by a small number of staff who were absent with serious illnesses.
He said the authority offered staff stress-management programmes, physiotherapy, smoking cessation programmes and self-managed walking, running and swimming groups.
He said: “Not all staff sickness absences result in cover by agency staff and these are limited to essential frontline posts such as care workers or social workers.
“These kind of jobs are often high-stress in nature and also involve high-impact work which is more likely to result in musculoskeletal injuries.”
A West Sussex County Council spokeswoman said: “The county council is committed to reducing levels of sickness absence and is working with managers and headteachers to do so. “ A Brighton and Hove City Council spokeswoman said the authority was reviewing agency use and cost.
She said: “On average the council spends £5 million a year on agency staff, so in 2011/12 spend was lower than average and in 2013/14 it was higher.
“This was due to a combination of service changes that needed temporary recruitment to make sure services continued to be delivered efficiently and cover for absences in essential posts, which have increased.”