Russia is adopting an "increasingly aggressive" approach to pursuing its foreign policy goals, including propaganda, spying and cyber-attacks, the head of MI5 has warned.
The Security Service's director general Andrew Parker said Russia had been a "covert threat" for decades but there were now more methods available for its agents to use.
In an unprecedented newspaper interview, Mr Parker said his service was working to disrupt the activities of Moscow's spies who were "at work across Europe and in the UK".
He told the Guardian that at a time when much of the focus was on Islamic extremism, covert action from other countries was a growing danger, with Russia the biggest concern.
"It is using its whole range of state organs and powers to push its foreign policy abroad in increasingly aggressive ways – involving propaganda, espionage, subversion and cyber-attacks" he said.
"Russia is at work across Europe and in the UK today. It is MI5's job to get in the way of that."
Mr Parker said Vladimir Putin's Russia "increasingly seems to define itself by opposition to the west and seems to act accordingly".
The spy chief said: "You can see that on the ground with Russia's activities in Ukraine and Syria. But there is high-volume activity out of sight with the cyber-threat.
"Russia has been a covert threat for decades. What's different these days is that there are more and more methods available."
His comments came as the Government prepared to unveil a new cyber-security strategy aimed at protecting the UK from online threats.
Mr Parker also warned about the threat posed by home-grown terrorists.
He said there were about 3,000 "violent Islamic extremists in the UK, mostly British".
The MI5 chief said his agency would expand from 4,000 to 5,000 officers over the next five years and dismissed suggestions that Brexit would hamper co-operation with European counterparts.
In what the Guardian hailed as the first newspaper interview given by an incumbent MI5 chief in the agency's history, Mr Parker explained why he was prepared to speak publicly.
"We recognise that in a changing world we have to change too. We have a responsibility to talk about our work and explain it," he said.
The Guardian published US whistleblower Edward Snowden's leaked revelations about the work of the intelligence agencies on both sides of the Atlantic and Mr Parker defended his criticism of the disclosure of the files.
"I spoke out at the time about the damage that was done to the work of British and allied intelligence agencies, about having so much about how we operate revealed to our adversaries. Secrecy is not something we need for its own sake," he said.
Earlier, Mr Parker warned the terror threat from Islamic State was "here to stay" and the group, also known as Isil, posed "at least a generational challenge".
He issued the stark assessment as MI5 revealed 12 plots had been foiled in the UK since June 2013.
In comments after an address to the Royal Society's annual diversity conference he said: "Isil is an enduring threat, here to stay, and is at least a generational challenge.
"MI5 and the intelligence agencies have good defences because of the investment made in our capabilities.
"We will find and stop most attempts to attack us, but not all."