By Angela Harrison
BBC News education reporter
A "sick note culture" is being blamed for an apparent rise in the sickness rate among schoolchildren in England.
Many thought the illnesses were fabricated
Research for Cambridge University found sessions missed due to sickness in a sample of 76 schools rose from 4.05% to 5.37% between 2002-03 and 2004-05.
Some head teachers and welfare officers said they thought parents were pretending their children were sick - or taking them on holiday.
The rise coincided with a government clampdown on term-time holidays.
Ministers had warned that parents who took children out of school without permission for holidays could face fines.
The government publishes figures on absence from school, both authorised and unauthorised, but does not give a break-down of the reasons. However, many local authorities do keep such records.
The national average for authorised absences is 5.85% of half-day sessions.
The research was led by truancy expert Ming Zhang.
He also asked 92 teachers or educational welfare officers why they thought there had been a rise in sickness rates.
Most thought the illnesses were genuine, but about 20% felt they were fabricated - and another 20% reckoned children were being taken on holiday.
Ming Zhang, who is the principal education welfare officer in the London Borough of Kingston, said some people blamed a "sickness note culture".
"The main problem with the current system is that you can't really tell whether a child is genuinely ill or not. You do need a doctor's letter to clarify that," he told the BBC News website.
"Yet it is not always possible for a parent to get a doctor's note without paying a fee.
"That's why education authorities and health services need to work together to find a solution."
A study of absence rates in Kingston had found a similar upward trend, he said.
The information from the survey - which is part of a wider study - would be useful in helping to find new ways of tackling the problem of school absence, he said.
A spokesman for the Department for Education and Skills said : "Every day in school counts, and we are challenging the culture that tolerates absence from school."
Attendance was now at record levels, he said. Most parents carried out their responsibilities to get their children to attend school effectively. "Naturally, there are children who have serious problems and parents who struggle to cope, and there is a significant package of investment and measures to give them every support, understanding and assistance to tackle the underlying causes of truancy."
The leader of the Secondary Heads Association, John Dunford, said the research suggested it might be useful to look more deeply into the authorised absence figures.
Anecdotally, head teachers report a big rise in families taking holidays in the second half of the summer term.
In Scotland, absence figures are now broken down into different categories.
Last year, short-term absences due to sickness ran at 1.4% of primary school sessions and 2.5% of secondary school sessions.
Statisticians noted that sickness rates increase suddenly when children start secondary school, particularly amongst girls.
Deprivation increased the likelihood of being absent due to sickness.