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Teachers have complained about "cheating watches" being sold online to give students an unfair advantage in exams.These digital watches include an "emergency button" to quickly switch from hidden text to a clock face.

The watches hold data or written information which can be read in exams.

But a deputy head from Bath has warned about the scale of this "hidden market" and says it could tempt stressed students into cheating.Watches are advertised on Amazon with the claim that they are "specifically designed for cheating on exams".

One model promises 4GB of electronic storage for text files or images, to be read on the screen of the watch.If the "emergency button" is pressed, the other buttons on the watch are disabled, as the watch switches to a conventional digital clock face.This "watch for easier studying" is also marketed as being compatible with a mini wireless earpiece.

Another advert promises a watch that is "perfect for covertly viewing exam notes directly on your wrist".Another is offering a new model for this year with 8GB capacity, which could hold video files.Smart watches or digital watches with concealed content would be covered by rules banning electronic devices from exam halls, such as mobile phones or any other computer technology.

But Joe Sidders, deputy head at Monkton Combe Senior School in Bath, has contacted the BBC to warn that the rise of small wearable devices risks becoming a "nightmare to administer"."I expect the hidden market for these sorts of devices is significant, and this offering on Amazon is just the tip of the iceberg," he said.

The deputy head says that it is irresponsible to try to sell such items to under-pressure pupils, who might get caught and disqualified from all their exams.He wants exam boards to take a tough line on this - and to challenge businesses making such devices available.

And he raises concerns that if such devices were in wide circulation it would call into question the validity of results.There is a model on Amazon on sale in the US for $61 (£43) which can be shipped to Europe. It is also advertised in the UK but described as currently unavailable.

Another advert for the UK market has a cheating watch for £44, but via a US-based seller. There are also "cheat pens" on offer, which allow students to conceal information inside a pen.Cheating watches are also advertised on eBay or can be bought online directly from retailers.

Concerns about the dishonest use of smart watches is already affecting examinations, according to people contributing to the BBC's Facebook page.

Eugene Beirne said he was currently taking airline pilot's exams and "one of the many things you are not allowed is a watch regardless of the make".Abigail Lauze said: "My microbiology professor does a watch check every time we have a test. If it's not an old school analogue it has to come off and go in the cell phone bin."

The Joint Council for Qualifications, speaking on behalf of exam boards, said schools should tell students "what is and what isn't allowed in the examination room"."A candidate found in possession of anything used for cheating - like these watches - would be reported to the awarding organisation whose examination was affected.

"This could result in the student being disqualified from the exam and the overall qualification."