1. ITV Report

GCHQ: From Cromwell to the internet

COLOSSUS, the world's first computer, invented to solve enciphered German messages. Photo: Crown Copyright reproduced with kind permission of Director, GCHQ

GCHQ (Government Communications Headquarters) is one of three UK intelligence agencies that form the UK's security and intelligence system. It is mandated by law to enact the UK's National Security Strategy, along with with Security Service (MI5) and Secret Intelligence Service (MI6).

It specialises in what is called "lawful interception" and has worked to intercept and decrypt messages for generations, for the purposes of national security.

Oliver Cromwell established the precendent that led to its formation, after signing an Act of Parliament that authorised the opening of selected items of mail:

To discover and prevent many dangerous and wicked Designs . . . the Intelligence whereof cannot well be communicated but by Letter.

– CGHQ website

The head of the GCHQ reports directly to the Foreign Secretary, and works with the Ministry of Defence, the Foreign and Commonwealth Office and police services.

GCHQ headquarters are in Cheltenham, Gloucestershire and the agency employs 5,300 staff, according to its website.

GCHQ employs more than 5,000 people in the UK. Credit: Press Association

Legal framework

GCHQ activities are underpinned by two main laws, The Intelligence Services Act 1994 and the Regulation of Investigatory Powers Act 2000. These stipulate the conditions and purposes for monitoring and interception, namely;

  • National security
  • Safeguarding economic well being
  • Prevention and detection of serious crime
The public/visitors entrance to the GCHQ in Cheltenham Credit: Press Association


The importance of interception methods became critical in the First and Second World Wars. With the invention of radio, a number of radio intercept stations were created, and role of cryptanalysts, linguists and radio traffic analysts became crucial to the war effort.

The most famous Sigint report of WW1 came from the decryption of a telegram sent to the German foreign minister in 1917 stating that the Germany intended to strike marine vessels trading with the British, and promising Mexico it would be granted Arizona, New Mexico and Texas if it joined the conflict. The telegram prompted the US to join the war.

The Zimmerman Telegram to the US authorities. A deciding factor in the USA joining the war effort, Credit: Crown Copyright reproduced with kind permission of Director, GCHQ

In World War Two, Bletchley Park became the sight of the intelligence agencies interception devices and played a pivotal role in the war effort. By the end of 1944, 10,000 people were employed at Bletchley park alone, with thousands more involved across the world.

The decryption of the Enigma code is the most famous achievement of the agency, but others include the development of COLOSSUS, the world's first computer designed to solve encoded German messages.

The COLOSSUS enabled allied forces to decrypt German messages, and share information in a classified way.

A letter from Churchill to GCHQ in 1940. Credit: Crown Copyright reproduced with kind permission of Director, GCHQ
One of the decoder devices situated inside Betchley Park Credit: Crown Copyright reproduced with kind permission of Director, GCHQ

Links with other agencies

GCHQ works in partnership with equivalent agencies all over the world - as allies through both world wars the US equivalent, the National Security Agency (NSA) are a key ally.

The NSA and GCHQ often work together, sharing information they gather and/or technology they develop. Information on the full extent of their collaboration is not readily available to the public.

One such project was the global electronic surveillance system code named ECHELON located near Munich in Germany. The site was controlled by the NSA and operated by the NSA in conjunction with GCHQ and their counterparts in Canada, Australia, and New Zealand. The NSA announced in 2002 they were closing the system.

The US National Security Agency and GCHQ worked together in ECHELON, closed in 2002. Credit: Reuters

More: GCHQ website

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