New figures reveal Lancashire splashed out £12.8m on supply teaching staff last year.
The county council is second only in England to Birmingham City Council, which spent £20.3m on agency workers.
The figures come as recent statistics reveal Lancashire has the highest rate of stress-related absence among teaching staff in England a
nd Wales.
Conservative County Coun Margaret Livesey is a member of the authority's Children and Young People Overview and Scrutiny Committee.
She said: "It does seem a lot and I would like to know the explanations and reasons. Supply teachers mean absenteeism.
"The size of classes and the pressures of teaching are making it very stressful, so people go off for stress-related complaints.
"No doubt it will be brought up at our scrutiny committee."
Coun Tony Pimblett, Lib Dem group deputy leader on the council, said: "It comes down to whether you can fill vacancies or not.
"It is the ideal situation to have all of our staff as full-time teachers but unfortunately, it's not always possible.
"One element is sickness which you can't legislate for and the other is teachers in an area may want to do part-time work."
Lancashire spent £12.7m in 2005/06 and £12.1m in 2004/05.
A spokesman for the council said of the figures: "We are the fourth biggest authority in the country so you would expect us to be near the top."
A total of £438m was spent on supply staff in England in 2006/07.
The figures were revealed by government Schools Minister Jim Knight in answer to a parliamentary question by Tory MP Anne Main.
Other statistics show 16,098 teaching days were lost in the county due to teacher illness.
County Councillor Vali Patel, cabinet member for schools, said:
" Lancashire has led the way in ensuring that schools get good value for
money from supply teachers.
"Five years ago we set up the Lancashire Teaching Agency in partnership with Reed, which has brough significant benefits for schools and supply teachers.
"Focusing on the total spend is misleading - as one of the largest authorities in the UK we naturally spend more in total than smaller authorities."
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