More Russian spies operate in the UK today than at any time during the Cold War, sources say.
Spooks believe up to 300 Kremlin agents are trying to steal state secrets – some with British nationality.
They say that is even more than during the Cold War, which started in the 1940s under brutal Soviet dictator Joseph Stalin.
Some espionage operations are run by personnel in the Russian embassy, but many more are done by civilians with no direct link to the Kremlin.
One intelligence source said: “Russia views the UK as a soft touch.
“Russian intelligence officers have conducted assassinations on UK soil, used nerve agents to poison people and endangered the lives of thousands.
“The Kremlin is also orchestrating daily cyber warfare attacks.”
Last October, MI5 head Ken McCallum said the spy threats posed by China and Russia to the UK are “growing in severity and complexity”.
And last week relations between the UK and Russia hit a new low when destroyer HMS Defender was almost dragged into a fight with Russians warships and jets in the Black Sea.
Russia said that one of its ships had fired warning shots while warplanes dropped bombs nearby.
The Ministry of Defence denied this and said the UK ship was in international waters.
But Russian president Vladimir Putin has threatened to bomb our ships if there is another confrontation.
Former Navy chief Lord West said Russia’s actions would have led to war in the days before nuclear weapons.
He added: “Putin has been incredibly reckless and stupid. The next time, there could be a mistake and someone could get killed and the situation becomes far more serious.”
Colonel Richard Kemp, a former Army chief, warned that Putin would not hesitate to attack our ships in future if they entered waters they considered their own.
He said: “Authoritarian regimes are not subject to the same limitations and constraints as we are and it would be a mistake to assess their potential response…