More than 100 people have sent in their answers to the six-stage conundrum since it was unveiled on the Cheltenham spy base's site last month.

It is the fourth time the public have been challenged to crack a code. The first answer was received just hours after the puzzle was posted online.

A spokesman said: "We have had an excellent response."

Visitors to the site are presented with a series of codes which represent extracts from written works.

Hidden word

The challenge is to decipher the codes, identify the work, and also find a hidden six letter word.

GCHQ plans to publish clues in August and provide the answers in September.

The spokesman said the response had already been so great there were plans to offer a prize next time.

But, perhaps unsurprisingly for such a shadowy organisation, the nature of that prize remains a secret.

"We are going to make it so that people will be keen to win," the spokesman said.