When a ring of Russian spies was discovered in the suburbs of several US cities in May, many commentators were quick to dismiss them as rather hapless.

There was, it seemed, a touch of comic opera about their covert activities, and the other details that emerged of their lives in the US - from the cultivation of hydrangeas, to the alluring pictures posted on Facebook by the most glamorous of the group, Anna Chapman.

But thats not how everyone saw it. Sir Stephen Lander, Director-General of MI5 until 2002, has told a BBC Radio 4 documentary, Why Russia Spies, that the very existence of a ring of Russian "illegals" (spies operating without diplomatic cover) is no laughing matter. 

"The fact that theyre nonde or dont look serious is part of the charm of the business," he says.

"Thats why the Russians are so successful at some of this stuff. 

"Theyre able to put people in those positions over time to build up their cover to be useful. They are part of a machine... And the machine is a very professional and serious one."

The use of illegals, says Lander, is a menacing type of espionage, perfected by the Russians during the Cold War.

"They were posted into the West with one of two roles," he says.

"One, to build up long-term cover with the eventual intention over many years to get a position in a government machine somewhere in the West, where they could spy for good. 

"The other role was to be a head agent of a network of spies who had been recruited by others, perhaps the legal residency, and were run from a third country by an illegal - still an intelligence officer, but not under any official cover."

To British intelligence, the fact that Russia is still prepared to fund and deploy illegals against the West is a cause for concern, not least because illegals are extremely difficult to uncover. 

Sir Gerry Warner, former deputy chief of the Secret Intelligence Service, MI6, says illegals are heavily deployed in Russias neighbouring states, like Ukraine and Georgia. 

"If they wanted to have illegals they could have them here," says Warner, "Ive no doubt about that. Whether they would think it worthwhile, I simply dont know."

Whether there are Russian illegals in Britain or not - and if there are, they are unlikely to be detected, Sir Gerry says - there is no doubt that "legal" Russian spies, those operating under diplomatic cover, are mounting an intelligence attack here. In fact, that attack is about as intense now as it was at the height of the Cold War.

"If you go back to the early 90s, there was a hiatus," says Lander. "Then the spying machine got going again and the SVR [formerly the KGB], theyve gone back to their old practices with a vengeance. 

"I think by the end of the last century they were back to where they had been in the Cold War, in terms of numbers."

What are those numbers? In the mid-1980s, during the Cold War, the Soviet embassy in Kensington and its trade mission in Highgate housed between 30 and 35 KGB officers, or their equivalents in military intelligence, the GRU, posing as diplomatic staff.

Together, they added up to about half of the USSRs diplomatic personnel in London. Today, the numbers are roughly the same, Whitehall sources believe.

Sir David Omand, a former head of the intelligence agency GCHQ, and Co-ordinator of Security and Intelligence in the Cabinet Office until 2005, says that although the level of Russian spying is at Cold War levels, the targets have changed.

"For the Soviet intelligence agencies there is that sense of momentum," he says, "that they just kept on going [after the Cold War]. But no doubt they switched their emphasis away towards economic targets."

Sir Stephen Lander agrees. "Theyre after things that bear on the strategic position of Russia, particularly its growing importance in the energy world," he says.

"So anything that gives them advantage in those areas would be worth stealing through spying. And the same applies to commercial developments and military developments."

Does Lander think Russian espionage is a serious long-term threat to Britain?

"Yes, I think it probably is," he says.

"A strong and peaceful Russia is in our interests. Its where they use covert and illegal means to leverage their position at the expense of the West that we need to have our eyes open."