The Secret Intelligence Service, MI6, is aiming to increase its headcount by nearly 1,000 by 2020.
The boost will take the spy agency's staff number from 2,500 to just under 3,500, BBC Newsnight reports tonight.
The move has been forced by the development of the internet and technology, it was reported.
The news, which came from Whitehall sources and has not been officially announced, comes a day after MI6 chief Alex Younger spoke of the scale of the challenge facing him.
"The information revolution fundamentally changes our operating environment," he said in a rare public appearance in Washington DC.
"In five years' time there will be two sorts of intelligence services – those that understand this fact and have prospered, and those that don't and haven't.
"And I'm determined that MI6 will be in the former category."