The NHS Counter Fraud and Security Management Service is looking into claims that a website called www.doctorsnotestore.com is offering sick notes in exchange for cash.
Bearing the signatures of real doctors and printed on NHS-headed paper, the documents come with a money-back guarantee if they fail to get employees out of work.
A spokesman for the NHS fraud squad acknowledged that selling sick notes is not illegal and those doing so cannot be prosecuted.
However, he added: "We take fraud against the NHS very seriously and anyone caught using these fraudulent documents will be pursued vigorously."
Speaking to the BBC, a spokesman from the British Medical Association condemned the activity and said improved relationships between employers and doctors could help to tackle it.
The Conservative group at Staffordshire County Council said recently that sick days at the local authority were costing taxpayers £22.5 million a year.
