Robert Hannigan has resigned as director of the Government Communications Headquarters (GCHQ) surveillance agency after just two years in the job.
Hannigan joined the civil service in 2000 and served in a number of senior, director-general level roles at the Northern Ireland Office, Cabinet Office and the Foreign and Commonwealth Office before being appointed to head up GCHQ in 2014.
Hannigan effectively acts as the permanent secretary for GCHQ, which is responsible for intercepting communications for the government and armed forces.
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In a letter to foreign secretary Boris Johnson published on the GCHQ website on Monday, Hannigan said he had decided to step down from the job for personal reasons, and after having "initiated the greatest internal change within GCHQ for thirty years".
"After a good deal of thought I have decided that this is the right time to move on and to allow someone else to lead GCHQ through its next phase," he wrote.
"I am, like you, a great enthusiast for our history and I think it is right that a new director should be firmly embedded by our centenary in 2019. I am very committed to GCHQ’s future and will of course be happy to stay in post until you have been able to appoint a successor."
Hannigan added: "I have been lucky enough to have some extraordinary roles in public service over the last twenty years, from Northern Ireland to No.10, the Cabinet Office and the Foreign Office. But they have all demanded a great deal of my ever patient and understanding family, and now is the right time for a change in direction."
Responding to Hannigan's letter, Johnson said the outgoing director had " led the renewal of some of our most important national security capabilities", and "set the groundwork for a major transformation of our cyber defences".
"Following your successful tenure, and thanks to the work of thousands of excellent GCHQ staff, the organisation is well placed to play its part continuing to protect our nation," the foreign secretary added.
Cabinet secretary Sir Jeremy Heywood also took to social media to pay tribute to Hannigan:
Thanks Robert Hannigan for leading @GCHQ superbly through recent challenges. Follows nearly 20yrs dedicated public service. Great colleague.
— Sir Jeremy Heywood (@HeadUKCivServ) January 23, 2017
According to the agency, an internal competition for Hannigan's sucessor will now be launched, with the current director and board continuing "to oversee all the department’s work". Hannigan's predecessor, Sir Iain Lobban, served as GCHQ director from 2008 to 2014.