Media playback is unsupported on your device Media caption The BBC's Gordon Corera says the code "may never be cracked"

Britain's top code-breakers say they are stumped by a secret code found on the leg of a dead pigeon.

The remains of the bird were found in a chimney in Surrey with a message from World War II attached.

Experts at the intelligence agency GCHQ have been struggling to decipher the message since they were provided with it a few weeks ago.

They say it may be impossible to decode it without more information - some of which could come from the public.

The message was discovered by David Martin when he was renovating the chimney of his house in Surrey.

Among the rubbish, he found parts of a dead pigeon including a leg. Attached to the leg was a red canister. Inside the canister was a thin piece of paper with the words "Pigeon Service" at the top and 27 handwritten blocks of code.

This was given to GCHQ at the start of the month.

"We didn't really hold out any hopes we would be able to read the message because the sort of codes that were constructed to be used during operations were designed only to be able to be read by the senders and the recipients," said GCHQ historian Tony, who asked that only his first name be used.

He told the BBC: "Unless you get rather more idea than we have of who actually sent this message and who it was sent to we are not going to find out what the underlying code being used was."

'Dear Santa'

The experts believe there are two ways the message might have been coded.

One is with a so-called one-time pad where a random "key" is applied to a message. If the key is truly random and known only to sender and recipient, the code can be unbreakable.

Another option is that this code was based on a specific - and now probably destroyed - code-book put together for a particular operation so the maximum amount of information about that operation could be relayed in the shortest message (this could be done in conjunction with a one-time pad).

There had been speculation that the message might have been sent by an agent of the Special Operations Executive and that it was heading for Bletchley Park. But these theories have largely been discounted.

An undercover agent in occupied Europe would not use an official note pad in case he or she was caught with it in their possession.

And Bletchley became a station to decode German and Japanese messages rather than a place in which the British military regularly sent its communications.

"The most helpful suggestion we had through all of this was from a member of the public who suggested that, since the message was found in the chimney, the first two words were most likely to be 'Dear Santa'," Tony said.

The best guess is that the message was sent by a unit in the middle of an operation in Europe which was on the move and so unable to stop and set up the aerial for a traditional wireless message.

It remains possible it could have been some kind of training exercise though - even perhaps for D-Day.

Secret to grave

GCHQ is on the lookout for any help in discovering the kind of contextual information that could aid the process by identifying the sender or recipient.

Based on the abbreviation of Sjt in the message, it is thought this was most likely an Army unit, since this is where the old fashioned spelling of Sergeant was used. But so far "Sjt W Stot" and X02 have not been identified.

Media playback is unsupported on your device Media caption The RAF regularly used pigeons as message bearers during WWII. Archive footage courtesy British Pathe

Another avenue is trying to identify the unit to which the pigeons referred to in the message were assigned.

Some 250,000 pigeons were used during the war by all services and each was given an identity number. There are two pigeon identification numbers in the message - NURP.40.TW.194 and NURP.37.OK.76. It is unclear which one relates to the bird in the chimney.

Help from the public is the best hope for any breakthrough.

"There are still quite a lot of people alive who worked in communications centres during the war and who might have some knowledge about this and it would be very interesting if anyone did have information if they could put it in the pot and we could see if we could get any further with it," explains Tony.

And without fresh information this pigeon may well have taken its secret to the grave.