Russia has resumed Cold War levels of spying and intelligence-gathering in Britain, senior Whitehall and security sources have revealed.

Among the spies are at least 32 Russian diplomats who are trying to obtain secret information about Britain’s military, technical and political capabilities, authoritative sources say.

The spy network is also collecting information about exiled opponents and critics of Russian President Vladimir Putin, a former KGB officer.

A Whitehall source said: “The level of espionage by the Russians in the UK is back to Cold War levels – it is business as usual for the spies.”

MI5, the domestic security agency, has warned Home Secretary David Blunkett of the growing Russian activities.

But the agency’s budget for counter-espionage work has been cut by half because resources have been re-allocated to deal with al Qaeda.

A confidential document, “Espionage Threat”, which was based on information provided by M15 and MI6, the Secret Intelligence Service, also reveals that Russian spies have been monitoring the movements of military aircraft in Britain.

The classified paper added that the agents were using the internet to target military specialists with access to secret information.

Several alleged Russian spies have left Britain, although these departures have not been made public to prevent a diplomatic rift with the Putin Government.

The build-up of the Russian spying capability in Britain and other European countries follows a change in policy adopted by Putin after he came to power in 2000.

The old Soviet culture of secrecy and foreign espionage, weakened under Boris Yeltsin, has returned.

Britain is of great interest to Russia because of its strong ties to the United States, its influential role in Nato and its attitudes towards Iraq and Iran.

As well, opponents of Putin living in Britain, such as Boris Berezovsky, are prime targets for the snoops.

The spy network is run by the SVR (Foreign Intelligence Service) – which partly replaced the old KGB – and the GRU, the military intelligence organisation.

The SVR has about 18 officers in Britain, and the GRU has about 14. All have diplomatic status, intelligence sources say.

Officers each have dozens of informers and agents. SVR is divided into three specialist fields, gaining intelligence on political issues, matters of security, and technology.

The GRU, which is considered the most hardline and active of the agencies and which also has dozens of agents, obtains information on issues such as Britain’s nuclear and military capabilities, and American bases.

MI5 and MI6 have given warnings about the growth of Russian espionage to Parliament’s intelligence and security committee, the MPs who oversee security matters.

The committee’s annual report this year said: “The threat from espionage did not disappear when the Cold War ended … countries such as Russia and China still want to acquire classified material and technology for exploitation by their own industry.”

It also noted that in 1999-2000 MI5 allocated 20 per cent of its budget to counter-espionage work – the bulk of which involved Russian spying – but this has dropped to 10 per cent.

Eliza Manningham-Buller, the director-general of MI5, told the committee: “There’s not less of it [espionage] about; we are doing less work on it. We are being more selective about the priority cases.

“It is something I have discussed with the Home Secretary. I recently gave him a summary and he is well-aware that we are carrying some risk here.

“The plan is to back-fill when we can but the international counter-terrorist work is moving and expanding at such a rate … “

Oleg Gordievsky, the double agent who was KGB head at the Soviet Embassy in London when he defected in 1985, said: “The strength of the KGB is that there are so many Russians living here and working for British companies.

“Every second Russian in a position of some importance is an informer to the KGB.

“The information is about individuals who might be of interest to the Russian authorities, and technology. They also want information about politicians.

“They have become much more active under Putin. Russia is under the foot of the KGB now.”

Alex Standish, editor of Jane’s Intelligence Digest, who has written extensively on this subject, added: “We are seeing an increase in the number of diplomats being posted in the UK … a significant proportion of these people are linked to the SVR.

“Putin is rapidly building up intelligence systems that were allowed to fall into decline under Yeltsin.”

A spokesman from the Russian Embassy in West London said: “No comment”.