Anyone caught using fake sick notes will be "pursued vigorously"
Fake sick notes are being sold on the internet for £25 each.
The documents on the website www.doctorsnotestore.com come on NHS headed paper and bear real doctors' names, but are completely false.
A site disclaimer says they are for "novelty" purposes only, but it also offers a money-back guarantee if they fail to get you out of work or school.
The NHS fraud squad said anyone using fake notes was open to prosecution and would be "pursued vigorously".
A spokesman said: "Selling sick notes is not illegal so a person could type one up and sell it without being prosecuted.
"It only becomes illegal when you receive a payment or advantage, for instance paid time off work, because of its use.
"We take fraud against the NHS very seriously and anyone caught using these fraudulent documents will be pursued vigorously."
The website advertises a guaranteed delivery time of 48 hours and an authentic NHS stamp - and is offering a buy-one-get-one-free deal.
Alongside the GP's name, the false certificate contains an illness of the buyer's choice.
The site also says: "Choose from doctors at medical centres in London, Manchester, Birmingham, Glasgow - or any other area."
It says it has been making fake professional documents for the UK, New Zealand, Europe and worldwide for 11 years.
False medical certificates, school notes, solicitors' letters and holiday insurance claims are also advertised.
A spokesman from the British Medical Association said: "The deliberate abuse of the sick certification system is to be condemned.
“One positive step employers can take to avoid fraud is to strengthen working relationships with doctors.
"This will also help them take positive action to support employees with genuine long-term health problems.”
BBC News has tried to contact www.doctorsnotestore.com but no-one was available for comment.