MI6 will grow from 2,500 people to close to 3,500 because of the increased strain placed on its operations by the internet and technological developments, according to BBC Newsnight.
The 2015 National Security Strategy and Strategic Defence and Security Review set out plans to employ 1,900 additional staff in the UK’s security and intelligence agencies.
No public announcement has been made as to how many of those extra employees would be allocated to MI6.
But the BBC says Whitehall sources have confirmed that MI6 is due to get the majority, with the rest to be shared out between the Security Service (MI5), GCHQ and police Counter Terrorism Command.
It comes amid claims security services are under “persistent” and sustained threat from terror groups like Islamic State (IS)
In a rare public appearance, the head of MI6 Alex Younger insisted Britain had made significant improvements in the way it tackled terrorism but admitted there was little sign of the “enduring” danger disappearing soon.
Appearing alongside international counterparts at a national security conference in the United States, the Secret Intelligence Service chief said technological advances presented both an “existential threat and a golden opportunity”.
Asked if the terror threat from groups like IS and al Qaida had reached its apex, Mr Younger said: “I would like to be optimistic about this but we have got quite long experience of this phenomena now and I see it very much as the flip side to some very deep-seated global trends, not least of all globalisation, the reduction of barriers between us.
“It’s a function also of the information revolution and the capacity for ideas to travel. It is fuelled by a deepening sectarian divide in the Middle East and there are some deep social economic and demographic drivers to the phenomenon that we know as terrorism.
“Allied with the emergence of state failure this means that, regrettably, this is an enduring issue which will certainly be with us, I believe, for our professional lifetime.”
At the conference, staged by the Central Intelligence Agency and George Washington University, Mr Younger said increased cooperation between agencies and improvements in the way governments tackled terror were “doing a great deal to mitigate the threat”.
IS, also known as Daesh, is struggling to hold on to stretches of the land it conquered when it surged through Iraq and Syria.
Mr Younger said terror groups take “ideological succour” from gaining territory and it was “good” that advances were being made to recapture it.
“But, I regret that I would have to forecast that whilst it is wholly desirable to remove territory, you will have a persistent threat representing some of the deep fault lines that still exist in our world that I think will be addressed by patience, long-term partnership and partnership that esteems that collective values that we defend,” he said.
Mr Younger said rapid technological changes “throw everything up in the air” and mean the service has to “fundamentally” look at the way we carry out intelligence operations.
“This is a fabulously important issue and one that will dictate our future success,” he said.
The MI6 chief said that the National Security Agency leak by so-called whistle-blower Edward Snowden in 2013 had been “highly problematic”.
Snowden took refuge in Moscow after revealing extensive internet and phone surveillance by the intelligence agencies.
“I think that the real issue for us has been the effect that this has had on the levels of trust between the intelligence communities internationally and the technology community where I think that the right and proper response to the common threats that face us is through community of effort and teamwork between those different groups,” Mr Younger said.
“And to the extent that those revelations damaged and undermined the trust that needs to exist, I think it is highly problematic.”