Schools in Oxfordshire have shelled out nearly £25m on supply teachers in just four years.

During the 2005/6 academic year - the latest period for which information is available - the county spent £6.6m on temporary staff, up from £5.2m in 2002/3. The total spent over the four-year period was £24.2m, Government figures show.

Part of the spending increase has been attributed to new arrangements aimed at reducing teachers' excessive workloads. Since September 2005, permanent teachers have been guaranteed about two hours per week away from the classroom to prepare for lessons and catch up on marking.

It has forced schools that are unable to afford extra full-time staff to choose between asking teaching assistants to take classes or paying for supply teachers to come in.

The National Union of Teachers (NUT) said the new workload agreement was designed to reduce instances of teachers working 54 or even 60-hour weeks. Although the agreement permitted teaching assistants to take classes, many schools chose instead to pay for qualified supply staff, something the NUT supported.

General secretary Steve Sinnott said: "We would prefer schools to hire teachers full-time, rather than paying money to supply agencies, but it's better that children are taught by qualified supply teachers rather than unqualified people."