Speaking to reporters at MI6’s headquarters in Vauxhall, central London, he said: “The connectivity that is at the heart of globalisation can be exploited by states with hostile intent to further their aims deniably. They do this through means as varied as cyber-attacks, propaganda or subversion of democratic process.

He went on: “The risks at stake are profound and represent a fundamental threat to our sovereignty; they should be a concern to all those who share democratic values.”

Mr Younger said the threat was posed by states “whose very survival owes to the strength of their security capabilities.”

British spies trying to defeat the threat faced complex and risky work “often with the full weight of the state trying to root us out,” he said.

Mr Younger also sharply criticised Russia’s policy in Syria of backing Bashar al Assad and said it risked driving Syrians towards extremists.

He said: “In defining as a terrorist anyone who opposes a brutal government, they alienate precisely that group that has to be on side if the extremists are to be defeated.

“Meanwhile, in Aleppo, Russia and the Syrian regime seek to make a desert and call it peace. The human tragedy is heart-breaking.”