A brain-teasing puzzle sent out in a Christmas card has hundreds of thousands of people stumped.

No one has been able to complete the puzzle put out as part of the UK's Government Communications Headquarters (GCHQ) traditional Christmas card.

GCHQ is a security and intelligence organisation tasked to protect the UK from threats and works closely with MI5 and MI6.

According to news.com.au the first stage of the challenge is a grid-shading puzzle and has been online since December.

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The puzzle consists of a grid of 25 x 25 squares, each of which must be black or white.

A series of numbers beside each row and column indicate how many black squares there are in that line, and how many occur together.

So "3 2 7" would mean that line has blocks of three, two and seven black squares, in that order, but without indicating where in the line they occur.

Each block of black squares must have at least one white space between them.

GCHQ revealed nearly 600,000 people successfully completed the first puzzle.

By solving the first puzzle people received a QR code that you could scan to be taken to the next part of the puzzle.

Four more separate puzzles follow and once all stages have been unlocked and completed successfully players were encourages to submit their answers to the GCHQ by January 31, 2016.

The winner was to be drawn from the successful entries.

While around 30,000 players reached the final stage, no one successfully completed the puzzle.

GCHQ Director Robert Hannigan said a number of people were "very close".

The complete solution will be published on the GCHQ website early this month.

- nzherald.co.nz