World's hardest sudoku: can you crack it?

But those who find our number puzzles a bit of a doddle may wish to sharpen their pencils for the ultimate test: what is claimed to be the world's hardest sudoku.

The Everest of numerical games was published by Arto Inkala, a Finnish mathematician, on his website and is specifically designed to be unsolvable to all but the sharpest minds.

On the difficulty scale by which most sudoku grids are graded, with one star signifying the simplist and five stars the hardest, this puzzle would score an eleven, he explained.

Sudoku is a familiar challenge to newspaper readers and puzzle enthusiasts, requiring each vertical line, horizontal line and nine-square box to contain every number from one to nine.

While that might sound simple, the particular difficulty in this version lies in the number of deductions you have to make in order to fill in a single number on the grid.

Instead of being able to spot where a number goes based solely on the boxes that have already been filled in, most moves will face you with two or more spaces where a number could fit.

Only one of these is correct, but to find it you must examine all possible options for your next move and perhaps the move after that, continuing in the same vein until all but one potential route results in a dead end.

Mr Inkala said the most difficult parts of the grid require you to think ten moves ahead, exploring a series of permutations at each stage in order to eliminate all routes other than the right one.

He added: "It is difficult to say if any one [puzzle] is the hardest or not, because I believe the hardest one is not yet discovered.

"I am not sure if it is impossible to make, but there are so many possibilities to formulate that [I think] the most difficult one has not yet been found."

Given up? Have a look at the answer.

A fan of Crossword puzzles? Try out our own Puzzles website where you can browse more than 15,000 mind-bending brain games to challenge and entertain you at puzzles.telegraph.co.uk.