More than one in every 100 secondary school pupils will miss classes because of chronic fatigue syndrome (ME) according to a children's charity.
Commenting on the results of a new study in the online journal BMJ Open, Mary-Jane Williams from the Association of Young People with ME said: "The impact [of ME] can be horrendous.
"One per cent is a phenomal number, it proves the problem we're up against but I would say that is on the lean side, there's a hidden population [with ME] out there."
Researchers from the University of Bristol looked at absenteeism amongst 2,855 pupils at three schools in Bath and found 28 missed at least 20 days in six-week term.
What they found equally worrying was that only five of those pupils had been diagnosed chronic fatigue and study author Dr Ester Crawley believes the national figure could be higher still.
"The results are only based on three schools and we only looked at those pupils who were missing the most school," she said. "Missing one day a week is severe, but the total level and impact [of chronic fatigue syndrome] is going to be higher."
While there is currently no universally accepted cure or treatment, 19 of the newly diagnosed pupils received cognitive behavioural therapy and 12 of those were now back in school full-time after six months.
These promising results prompted Professor of Psychiatry Matthew Hotopf to say "this demonstrates about two-thirds recover by six months so there is everything to play for in terms of outcome for chronic fatigue syndrome in kids."