Alexander Litvinenko's death put the spotlight on the world of spying
The sources told the BBC's Frank Gardner there were more than 30 identified intelligence officers trying to get secrets by covert means.
Targets include military hardware, scientific know-how and technology, and inside tips on Westminster politics.
Businessmen who may have access to sensitive information are also of interest, as are Russian dissidents.
Such dissidents include Boris Berezovsky, friend of the murdered former KGB agent Alexander Litvinenko.
Sir Paul Lever, a former member of the Joint Intelligence Committee, said: "Russian espionage activity in Britain is very extensive.
"In scale it's probably pretty much as it was at the height of the Cold War."
He described the activities of Russian spies and how they operate.
"Mostly it is by gradually drawing their targets into a relationship which at first may seem to be a perfectly normal business relationship - 'we'd like some information, we'd like an article, of course we'll pay you', but over time develops into something more akin to the classic relationship of case officer and agent."
Once a potential agent has been hooked, his or her Russian handler then needs to avoid the British authorities.
In Britain, counter-espionage has been carried out by the security service M15 for nearly 100 years.
The service now focuses mainly on counter-terrorism and only about 5% of its budget is now spent on counter-espionage.
The service oversees every visa application from a potential intelligence officer coming to Britain - and tries to keep out the more experienced ones.
Those that do get in usually operate from the Russian embassy in Kensington. Here they report to a controller, known as "the resident".
But keeping tabs on these agents is a difficult task.
Surveillance expert Crispin Black said: "You need large numbers of people to keep them under round-the-clock surveillance.
"And you have to change those people every now and again. Say the suspect goes on a journey - you can't follow them from here to Glasgow in one white Peugeot up the M1."
Not only that, but our security correspondent said that some Russian intelligence officers were reappearing in London from 20 years ago.
In Russia, Britain is in turn accused of spying.
Last year saw the case of "the rock" - allegedly a device planted by MI6 to receive coded data from agents.
Russian security expert Andrei Soldatov said: "Now Russian counter-intelligence thinks that British intelligence are not only spying against Russia but trying to influence the political situation."