Read GCHQ director Richard Hannigan's resignation letter in full

Robert Hannigan, the director of the British intelligence agency Government Communications Headquarters (GCHQ), is to step down from his role.

Hannigan, who was appointed chief of GCHQ in April 2014, said he will formally step down when his successor is announced.

He explains that this is for personal reasons in a letter to Foreign Secretary Boris Johnson.

The letter can be read in full below:

Dear Foreign Secretary,

I have had the great privilege of leading the men and women of GCHQ since 2014. I am proud of what we have achieved in those years, not least setting up the National Cyber Security Centre and building greater public understanding of our intelligence work.

I am equally proud of the relentless 24 hour operational effort against terrorism, crime and many other national security threats. While this work must remain secret, you will know how many lives have been saved in this country and overseas by the work of GCHQ. Underpinning this is our world-class technology and, above all, our brilliant people. As you know, I have also initiated the greatest internal change within GCHQ for thirty years, and I feel that we are now well on the way to being fit for the next generation of security challenges to the UK in the digital age.

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. 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.

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.

I want to thank you and the many Ministers I have served over the years, and to thank the Prime Minister and her three predecessors, for the opportunities I have been given. I have worked with outstanding people, whether politicians or Crown Servants, and none more so than in GCHQ. I am very confident that they will continue to achieve even greater things in the future.

With very best wishes.