Legal health experts have issued a warning about fake sick notes after noticing an emergence of replica doctor's letters available to buy online.
One website offers a "real actual (replica) doctor's note" for £9.99.
Other websites offer advice on how to forge a note.
The Medical Defence Union (MDU) has urged doctors to look out for fraudulent documents.
Medico-legal adviser Dr Ellie Mein said the MDU had supported members with cases concerning a number of false notes - from forged prescriptions and genuine sick notes that have been altered to prolong the duration of the patient's sick leave, through to more elaborate letters that have been created entirely from scratch.
"With editing software freely available it has become increasingly easy to make a convincing copy of a practice's headed notepaper," she wrote in the MDU journal.
"There are even various sites online that offer advice on how to convincingly forge a sick note or that allow you to buy replica NHS sick notes."
In one case detailed in the article, a GP received a phone call from a university seeking clarification on a note asking for a student's deadline to be extended due to ill-health.
A review of the patient's notes showed they had not been seen for at least two years. But the letter was written on paper with the practice letterhead on it with a similar signature to the GP's.
The MDU has issued advice on the next steps medics should take if they encounter such an incident.