Britain's intelligence agencies are engaged in a "technology arms race" with terrorists, cyber criminals and other "malicious actors" bent on causing the country harm, the head of MI6 has warned.

Alex Younger said the agencies were facing opponents "unconstrained by consideration of ethics and law" who were exploiting internet technology to "put our people and agents at risk".

In his first public comments since taking over last year as Chief of the Secret Intelligence Service (SIS) - as MI6 is more properly known - Mr Younger said traditional human espionage was becoming increasing intertwined with "technical operations".

Speaking to an invited audience in London, he said that using the internet and "big data" had enabled the agencies to "sharpen some very human characteristics" of their work.

"Using data appropriately and proportionately offers us a priceless opportunity to be even more deliberate and targeted in what we do, and so to be better at protecting our agents and this country," he said.

However he said that technology had also created new vulnerabilities which their opponents were able to exploit.

"The bad news is the same technology in opposition hands, an opposition often unconstrained by consideration of ethics and law, allows them to see what we are doing and to put our people and agents at risk," he said.

"So we find ourselves in a technology arms race. Contrary to myth, human intelligence operations are not an alternative to technical operations - the two are interdependent and set to become more so."

Mr Younger described the threat faced by agencies as the "dark side of globalisation" - including "terrorists, malicious actors in cyberspace and criminals" as he paid tribute to the bravery of his officers.

"As I speak there are SIS officers serving in some of the most dangerous and forbidding places on the planet," he said.

"Others are operating under deep cover, unable to reveal the real nature of their work, or sometimes even their identity. This takes a particular type of bravery and resilience."

He added: "I am particularly proud of the way in which our work with the military developed in Iraq and Afghanistan. Put bluntly, work done by SIS and GCHQ saved many British and coalition lives."

Mr Younger also defended the system of parliamentary oversight by the Intelligence and Security Committee - despite frequent criticism that it is ineffectual - insisting it was essential to maintaining public trust in the work of the agencies.

"What really distinguishes us from our opponents is that we live by the values of this country and are regulated by its laws, even as we work in secret. This is our vital advantage," he said.

"Our staff are asked to make complex decisions in a difficult ethical and legal space. They do so with remarkable assurance. If we make mistakes, we face up to them and learn from them.

"The guiding principle is clear - we cannot protect the values this country represents if we undermine them in the process. And we cannot hope to hold the public's trust unless they know this principle is effectively overseen."