The writer behind the first authorised history of GCHQ said it was “essential” to provide a “warts and all account” of its 100-year past, documenting its successes and failings.

John Ferris was given unrivalled access to the security agency’s archives in order to write Behind the Enigma.

He told the PA news agency about some of his other findings:

– The secrets that remain

Mr Ferris has signed the Official Secrets Act and is therefore forbidden to discuss some of the information he has been privy to.

He was not allowed to view any information on diplomatic communications or intelligence after 1945, and also information on technical issues which could be relevant still today.

While GCHQ ended up giving him “more material than they promised”, other intelligence agencies – in America, Canada and Britain – were entitled to be informed of material being written about them by an allied organisation and, if necessary, could request they were not mentioned or have excerpts removed.

Mr Ferris said: “That took a long time to negotiate. A fair amount of what I wrote was removed.”

He thinks about 5% of his work was removed, but the main story has still been told.

He also said he decided not to go into details of material that had been leaked from GCHQ over the years because it would need space to explain, would be “hard to stop” and would go beyond the parameters of what had been agreed.

– The staff

The book talks of how a GCHQ employee in 1945 discovered through Sigint – intelligence gathering by interception of signals – that his son had been killed in action, but he was not allowed to discuss his son’s death with his wife.

Mr Ferris described how his research showed staff were bound to a life of secrecy, but were used to it.

He said: “They all internalise this idea that they should not talk about what it is they do. And not even tell members of their family and they simply get used to it.

“You have mathematicians who make a discovery which is five or 10 years ahead of what any civilian mathematician does and yet they can’t publish it.

“Now for most mathematicians that would be very frightening, horrifying, but they are used to the idea.

“They see themselves really as being given the opportunity sometimes to use leading-edge kit to do fascinating work. They are willing to live without getting the personal credit.”

He also described the GCHQ’s union ban in 1984 as “tragic” because it affected a third of the organisation which worked in intercepting foreign communications.

A promotion in such roles was “virtually impossible” and there were strikes in a bid for better conditions, but GCHQ decided a union ban was needed over fears walkouts could affect national security and to ensure they could maintain long-term co-operation with the Americans.

The staff had no choice as they could otherwise be faced with job losses.

– Changes

Mr Ferris said the security agency has gone from being “an absolutely secret organisation, which tries to hide its existence and work from people, to being an organisation which in fact is constantly in the news, constantly talking to British people about problems and threats and speaking more openly to the British people than I think the heads of any other British security or intelligence organisation”.

He thinks GCHQ has now “struck the right balance” between remaining confidential and being more open.

The book is also reveals:

– Failures in British signals security in the First World War contributed to tens of thousands of British deaths at the Battle of the Somme, due to German interception of British military messages on field telephones.

– The first example of British signals deception blindsiding German general Erwin Rommel during the Second World War through a British-controlled agent, codenamed “Cheese”, GCHQ passed misleading and fictitious reports between July and October 1942.