Russia has hit back at the head of MI5's claims that Moscow is adopting an "increasingly aggressive" approach to pursuing its foreign policy goals, including cyber-attacks.
Britain's Security Service director-general Andrew Parker used an unprecedented newspaper interview to warn that Russia had been a "covert threat" for decades but there were now more methods available for its agents to use.
He said Russia is carrying out a "high volume" of activity "out of sight", such as cyber- attacks, but the claims were dismissed by Kremlin spokesman Dmitry Peskov.
"His (Mr Parker's) words do not correspond to the reality," Mr Peskov said.
"We do not agree with them, and claims regarding cyber-attacks we have already commented on.
"No-one has yet given any evidence so we cannot consider claims that are not founded on evidence."
Mr Parker told the Guardian that MI5 was working to disrupt the activities of Moscow's spies who were "at work across Europe and in the UK".
The interview came as the Investigatory Powers Bill, dubbed a "snoopers' charter" by critics, returned to the House of Commons in the UK on Tuesday so MPs could consider amendments made by peers.
Ministers are determined to complete the Bill's passage through the British Parliament by the end of the year, when many of the spying powers in the compromise Data Retention and Investigatory Powers Act 2014 expire.
Elsewhere, British Chancellor Philip Hammond unveiled the British Government's new cyber-security strategy, insisting the UK would "strike back" if it comes under attack.
Mr Parker said 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 President 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."
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.