Britain must spend more on its defense budget in order to be able to protect itself and its allies from the increasing threat posed by Russia, the former head of the foreign intelligence service said.
John Sawers, who left the Secret Intelligence Service (SIS) in November, told the BBC Britain must also increase dialogue with Russia.
"We've got to have the capability to deal with things like the hybrid warfare that we've seen Russia deploy, first in Crimea and then in the Donbass region, we've got to have the ability to deal with cyber-warfare," Sawers said.
"We're going to have to spend more on our defense and our security because the threats are greater."
Pro-Russian rebels have seized territory in eastern Ukraine.
Kiev, the West and some Russians accuse Moscow of sending troops to support the rebels, an accusation Russia has denied.
"They keep on reminding us that they have nuclear weapons," Sawers said "We shouldn't kid ourselves that Russia is on a path to democracy because it isn't."
Last week Britain scrambled Typhoon fighter jets to see two Russian long-range Bear bombers off the south coast of England, the second such incident in as many months.
British Defense Secretary Michael Fallon has said Russian President Vladimir Putin poses a "real and present danger" to Estonia, Latvia and Lithuania and NATO was getting ready to repel any possible aggression.