Teachers took more than a quarter of a million days off sick in Wales last year, according to Assembly government figures.
Full-time teachers took more than 220,000 sick days in 2008, the highest number for six years.
Part-time teachers took 30,000 sick days, the largest number since they were first included in government statistics in 2002.
Overall, teachers in Wales took just over 253,900 sick days last year, almost 9,200 more than in 2007 and averaging nine days for every teacher in Wales.
Despite this, the proportion of teachers off sick dropped slightly, from 65 to 64 per cent, meaning that the rise in absences was due to staff taking longer to return to work.
Teachers' unions blamed the longer absences on rising workload, which they said had led to increased stress and mental health problems.
David Evans, secretary of NUT Cymru, said: "We are concerned at how many of these days off are work related. There might be a lot of people off because of stress and workload issues that have still not been properly addressed."
Rex Phillips, Wales organiser of NASUWT, said: "Teaching is an extremely stressful profession. Those teachers who are off sick are off for longer, which highlights the stressful nature of the job."
Hannah Essex, of the Teacher Support Network (TSN), said more than half of the calls it received were from teachers experiencing common mental health issues, such as anxiety or depression.
The number of teachers in Wales who contacted the charity's helpline or website for support has increased sharply in recent years.
Ms Essex said: "Stress is now the major cause of sickness. It can lead to physical complaints such as headaches, lack of sleep and chest pains. Teachers are still experiencing extensive workloads and long hours. Increasingly, they are having money problems.
"These new statistics show that there's more of a need for our services than ever."
Mr Evans said local authorities and the government must act quickly before new rules on covering absent colleagues are introduced in September.
"The introduction of the `rarely cover' rule makes it even more important for schools and authorities to analyse the reasons for sick days and see what they can do to prevent them and reverse the trend," he said.
"We want a happy workforce that is able to cope with the demands of the teaching profession. It would appear that we are some way off achieving that."
Mr Phillips said the statistics called into question some local authorities' "punitive" sickness absence policies.
He urged the government to "fully embrace" workforce remodelling. "The remodelling agenda really needs to be stepped up so teachers can fully benefit from a decent work-life balance and proper planning, preparation and assessment time," he said.
A spokeswoman for the government said it recognised the importance of meeting the professional needs of teachers so they could be better supported in their roles.
"We are committed to the full implementation in Wales of the national agreement on raising standards and tackling workload, which is aimed at reducing teachers' workload and improving their work-life balance," she said.
"We are also continuing to fund Teacher Line Cymru (part of the TSN) this year to provide a free counselling, support and advice service for teachers in Wales."
The statistics also show that vacancies have risen slightly, from 94 in January 2008 to 97 this year, with Welsh and maths suffering the greatest shortages.
The number of full-time equivalent teachers was 28,333 in January, a fall of 723 on last year.
Teacher Support Network Cymru, phone 08000 855088.
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