Never mind Enigma - we couldn’t even finish the crossword

The 29-year-old said she and a five-strong group of colleagues - including Benedict Cumberbatch - who portray encryption specialists in The Imitation Game were still unable to finish it even after all that time.

She plays Joan Clarke, part of the team based at Bletchley Park in the Second World War who were tasked with unscrambling messages sent by the German forces using Enigma machines.

Knightley admitted that even when a maths expert came in to give the cast an explanation of the code-solving work, she struggled to grasp it.

“It was that feeling I haven’t had since school being in maths, of you sort of feel like you’ve died, of not being able to concentrate at all.

“And I desperately wanted to because he was such a nice man, and I was thinking, ‘This is really interesting, I should be paying attention.’

“But I couldn’t.”

Her character Clarke was among the recruits to the secret project who had actually been brought on board after showing their ability by completing a cryptic crossword.

Knightley said: “One day we decided we should all really do the crossword. So we got the quick crossword, there were five of us, it took us five days, and we still didn’t finish it. We were really bad at all of it. And I didn’t understand any of the maths.”

The Imitation Game premieres tonight at the opening of London Film Festival in London’s Leicester Square.