Amateur cryptographers have just a few hours to solve GCHQ's head-scratching Christmas puzzle, with the deadline for the festive challenge coming at midnight on Sunday.
Robert Hannigan, director of GCHQ - one of Britain's intelligence and security agencies - released a festive card last month featuring not reindeer or Father Christmas, but a cryptographic challenge.
Participants had to fill in a grid-shading puzzle to unveil a picture, which is the first in a series of increasingly complex challenges.
Nearly 30,000 players have reached the final stage - about 5% of those who started it - but none has successfully completed it, GCHQ said this week.
The complete solution to all stages of the puzzle will be published on the GCHQ website in early February after all entries have been received and the competition has closed.
GCHQ's website attracted huge visitor levels eager to take the challenge over the Christmas period. Nearly 600,000 people successfully completed the opening stage of the puzzle - a nonogram which when completed correctly, creates a "quick response" code which leads to the next stage of the challenge.
The puzzle was designed to amuse recipients of the director's card, but also encouraged them to make a donation to his chosen charity this year, the NSPCC.
Offering the tiniest glimmer of hope to frustrated puzzlers, he added: "With a few days to go no one has cracked it all yet, so my one and only clue is: it's not as abstract as you think."