Russia under President Vladimir Putin is potentially the "single greatest threat" to Britain's security, Foreign Secretary Philip Hammond has warned.
In some of the strongest comments by ministers since the crisis in Ukraine, Mr Hammond said the Russian leader had decisively rejected efforts to draw his country into a "rules-based" international order and was now actively seeking to subvert it.
He said that in the face of the "increasingly aggressive" stance of the Russian military, the effort to establish its intentions was now once again a "vital" element of the work of Britain's intelligence agencies - MI5, MI6 and GCHQ.
"The rapid pace with which Russia is seeking to modernise her military forces and weapons combined with the increasingly aggressive stance of the Russian military including Russian aircraft around the sovereign airspace of Nato states are all significant causes of concern," he said in a speech to the Royal United Services Institute in London.
"We are in familiar territory for anyone over the age of about 50, with Russia's behaviour a stark reminder that it has the potential to pose the single greatest threat to our security.
"Hence, continuing to gather intelligence on Russia's capabilities and intentions will remain a vital part of intelligence effort for the foreseeable future. It is no coincidence that all of our agencies are recruiting Russian speakers again."
His comments come after Defence Secretary Michael Fallon warned last month that there was a "real and present danger" that Mr Putin could try to de-stabilise the Baltic states - Latvia, Lithuania and Estonia - which, unlike Ukraine, are all members of Nato.
Mr Hammond said that Western countries now had to accept that their attempts since the end of the Cold War to draw Russia into the international order had been rebuffed.
"We are now faced with a Russian leader bent not on joining the international rules-based system which which keeps the peace between nations, but on subverting it," he said.
"President Putin's actions - illegally annexing Crimea and now using Russian troops to destabilise eastern Ukraine - fundamentally undermine the security of sovereign nations of Eastern Europe."
During a question and answer session following his speech, Mr Hammond hinted at the possibility that Western powers could seek to expose publicly the private financial arrangements of senior regime figures outside the country in order to embarrass them in front of their own people.
"It is a very interesting thought," he said.
Appearing later before the Commons Foreign Affairs Committee, Mr Hammond indicated Britain could be prepared to start supplying arms to the Ukrainians if their armed forces appeared to be crumbling in the face of the onslaught by the Russian-backed separatist rebels.
The Government has so far restricted its assistance to Kiev to non-lethal equipment, and Mr Hammond that it remained wary about going further, despite calls from some Tory MPs to arm the Ukrainians directly.
"We want to keep our options open here but we don't believe there is a military solution to this conflict and we are very wary of giving the mis-impression that we perhaps do think that if we were to focus on supplying lethal equipment to the Ukrainians," he said.
"But equally, we can't afford to see the Ukrainian armed forces crumble."
He warned that any renewed offensive by the separatists would trigger a fresh round of European sanctions against Moscow.
"There is a very clear acceptance across the European Union .... that if there is a significant breach of the ceasefire - a major assault for example on Mariupol - the European Union would have to respond and respond immediately with a significantly increased regime of sanctions."
He said the Government fully understood the concerns in the Baltic states that they could be Mr Putin's next target, and he made clear that the convention that ministers should consult Parliament on military action did not mean it could not act while Parliament was dissolved for the general election.
"In no circumstances would it be right to postpone military intervention that was required for the safety and security of Britain or the alliance because we were unable to consult Parliament because it was dissolved at the time," he said.
"It would require the government to bring that issue to Parliament as soon as the new Parliament was formed for what would be retrospective endorsement."
Mr Hammond also dismissed suggestions that Britain had been sidelined in recent diplomatic efforts to resolve the crisis, saying that German chancellor Angela Merkel had taken the lead for the EU as the leader who had the "closest thing to a working relationship" with Mr Putin.
"Our view was always that the Russians wouldn't have agreed to have the conversation if we and/or the Americans had been included," he said.
"We have played a very significant role, if you like, playing the bad cop. We have focused on stiffening the resolve of the European Union on sanctions, working using the resources of our own intelligence agencies to identify targets for sanctions."