A man who spent a decade combating Northern Ireland terrorism has been appointed as the new director general of MI5.
Ken McCallum takes over from Sir Andrew Parker, who retires in April having been in charge of the organisation since 2013.
Mr McCallum has 25 years' experience in MI5, and has also worked on Islamist extremism and cyber security.
He said: "MI5's purpose is hugely motivating.
"Our people - with our partners - strive to keep the country safe, and they always want to go the extra mile.
"Having devoted my working life to that team effort, it is a huge privilege now to be asked to lead it as director general."
Sir Andrew said: "Ken will make an outstanding director general.
"He's the right person to take MI5 forward and is already leading work to shape the future of MI5.
"He brings a wealth of leadership and national security expertise to the role, which is all the more important today as MI5 and the country deals with the impacts of the coronavirus pandemic."
Mr McCallum's career has seen him take charge of the MI5 response to the attempted assassination of former spy Sergei Skripal in the Salisbury nerve agent attack.
He has been deputy director general of MI5 since April 2017, with responsibility for all of MI5's operational and investigative work.
Mr McCallum led the Security Service's strategic response to the 2017 terrorist attacks.
In 2012 he took charge of all counter-terrorism investigations and risk management in the run-up to, and during, the London Olympics.
Mr McCallum began his career in MI5 with 10 years focused on Northern Ireland-related terrorism.
He grew up in Glasgow and holds a maths degree from Glasgow University.
MI5 said his out-of-work life is dominated by "trying to be an active father" and he enjoys mountain hiking.
Home Secretary Priti Patel welcomed the appointment, and paid tribute to Sir Andrew, the outgoing director general.
"Since becoming Home Secretary, I have worked closely with Ken and I'm delighted to appoint him as the new director general," she said.
"We are facing unprecedented national security threats and I'm confident that his experience and vision will allow the UK to meet those challenges head on.
"I also would like to pay tribute to Sir Andrew Parker, who has led the service through a very challenging period which saw the threat to the UK evolve rapidly.
"He has served his country with dedication and commitment and I thank him for his efforts."
Cabinet Secretary Sir Mark Sedwill said: "Ken McCallum's expertise and leadership will be crucial to ensuring that the Security Service remains agile and creative in the face of new and emerging threats to our security.
"I know that he will be a fantastic director general and I look forward to working with him."