At first glance, the Christmas card looks no different from any other; seasonal decorations, a festive red-nosed reindeer, a robin, a plum pudding, a cocktail, crackers and baubles.

Upon opening the card, however, it becomes clear that whoever sent it has gone to great lengths to conceal their identity.

There are no seasonal greetings or signatures - merely a mysterious drawing of a magnifying glass.

The card is the latest in a series of puzzles set by the Secret Intelligence Service, or MI6, to test the Christmas code-cracking skills of British spies, among others.

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And, if you spot it on the mantelpiece at a relative's home this year, have no doubt they either have friends in the highest echelons of Britain's secret service or are themselves a spy.

Hidden in each drawing is a fingerprint containing written clues that can only be seen under intense magnification.

Each cluster of words contains sentences that should help the recipient eventually solve the riddle of who the well wisher is.

The clue concealed in the Christmas cracker's grey shading reads: "Try the robin, just a cracker, no joke."

The tiny writing, invisible to the naked eye, on the robin's breast states: "Unfortunately I'm not the message. Try looking at the plum pudding."

The fingerprints in each drawing reveal a hidden message. Photo / Telegraph
The fingerprints in each drawing reveal a hidden message. Photo / Telegraph

And, the microscopic swirl of words on the Christmas pudding says: "There's no silver sixpence in me. You may have better luck with the red nose reindeer."

The shaded wording concealed in the deer's face reads: "You definitely need to look at the olive... but that might be a little too obvious."

That is perhaps a gentle nod to James Bond's favourite tipple of vodka martini (shaken not stirred), which has an olive garnish.

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However, the message hidden in the cocktail's olive sheds no light on the solution to the puzzle: "I'm a red herring. Try something else."

The bauble, on the other hand, requires the most painstaking analysis. When viewed through a magnifying glass the message clearly says: "You may have found the clue, just look a little closer."

Careful inspection of every minuscule line of writing unearths the sentence: "Take a look at the back of the card."

The martini may be a gentle nod to James Bond's favourite tipple of vodka martini (shaken not stirred), which has an olive garnish.
The martini may be a gentle nod to James Bond's favourite tipple of vodka martini (shaken not stirred), which has an olive garnish.

And finally, the back of the card features the tiniest of green fonts where it is just about possible to read: "You've found the secret message. Well done. Wherever you are in the world, have a wonderful Christmas and a prosperous New Year. From your friends in the Secret Intelligence Service."

But even then, the riddle is not completely solved. The green colour of the font holds an even bigger clue to who sent the card.

Green ink has traditionally been used by the chief of MI6.

The world's oldest intelligence agency was founded by Captain Sir Mansfield Smith-Cumming, the legendary spy-master, in 1909, and was then called the Secret Service Bureau.

He brought with him the naval tradition of using green ink to write official documents, as well as signing off his secret missives with a green letter 'C' for Cummings.

Ever since then, the chief of the SIS - currently Sir Alex Younger - has written in green ink, with the letter C now standing for chief.

The secret service Christmas card has MI6's fingerprints all over it. Photo / Telegraph
The secret service Christmas card has MI6's fingerprints all over it. Photo / Telegraph

It is even possible that seasoned spymasters or cryptographers will be able to decipher a far deeper meaning hidden in the apparently random placement of red and green dots and black circles on the card.

The SIS website explains that "we're secretly just like you" and says "our people work secretly around the world to make the UK safer and more prosperous".

It adds: "We are creative and determined - using cutting-edge technology and espionage."

In 2009, The Daily Telegraph's cartoonist, Matt, was asked by Sir John Scarlett, then the chief of the British Secret Service, to design a Christmas card for MI6.