Tens of thousands of Russian spies are living in Britain, a major report which has been endorsed by the former head of MI6 has warned.

A new study from the Henry Jackson Society thinktank said there were as many as 75,000 informants in London - half the 150,000 Russians who live in the capital - who are actively helping Moscow's secret service.

The study also warned of a huge increase of Russian case officers in the UK over the past eight years, with 200 officers handling an estimated 500 agents.

And the report estimated that as many as half of Russian Embassy diplomats are actively engaged in intelligence work.

The report said the number of spies now comfortably outstrips Cold War levels. Oleg Gordievsky, the KGB colonel who was a double agent for MI6, estimated there were 39 Soviet case officers in 1985.

The report was backed by Sir Richard Dearlove, the former head of MI6 credit: Martin Pope

The modern spies could be either some of the staff working at the embassy in London or covert operatives such as those using false identities to pose as Britons or Russian nationals living openly in the UK.

The report said: “The primary task for Russia’s spies is to gather intelligence on individuals who currently occupy, or previously occupied, positions of influence and power, particularly those who are consequential to Russian affairs.”

It added: “Russia’s embassy in Kensington currently maintains a staff of 56 diplomats, around half of whom, government sources suggest, are believed to be engaged in intelligence work – both declared and undeclared.”

The society called for the Government to redouble its efforts in combating the threat posed by Russia.

It also asked ministers to examine whether the Government should assess whether to revoke the Parliamentary access of media outlets “playing host to Russian spies”.

Andrew Foxall, the report’s author, said the study “shines a light upon the activities of Russia’s cloistered intelligence agencies in Britain. The threat they pose harks back to a darker age”.

He added the Russian spies are “not just engaged in assassinations” in the wake of the Skripal poisoning in Salisbury.

“Instead, they are busy delivering Russia’s full suite of “active measures” as part of a broad and malevolent effort to undermine our society," he said.

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“For too long, Britain’s security policy has been premised upon the naïve assumption that Russia has given up its Cold-War mind-set.

“This report shows that Russia’s spycraft is just as audacious as it has ever been.”

Sir Richard Dearlove, the former head of MI6, backed the report, saying that it “forcefully reminds us that Russian intelligence activity in the West is still large scale and intrusive, and that we need to devote significant resources and expertise ourselves to monitoring and blunting this threat to our national security.

“As during the Cold War, an effective counterintelligence capability remains an essential part of our own intelligence and security community.”

The Russian embassy in London dismissed the study as "political paranoia" in a posting on its Twitter feed.