|This compromises technology cooperation with Britain.
LONDON - BRITISH officials have warned that having to counter Russian spies in the country will compromise the fight against Islamist terrorist plots.
They also said the chances of these terror plots succeeding have risen significantly.
The warning comes in the wake of revelations last week that Russia is now considered the third most serious threat facing Britain.
The 'league table' of threats to the nation's security is headed by Al-Qaeda terrorism, with Iran's nuclear proliferation in second place.
Britain has raised the problem posed by Russian agents at diplomatic levels, but the concerns have been dismissed.
'The government has spoken to Moscow and asked them to stop but their response is 'everyone spies on everyone',' said one senior security source.
'MI5's resources have been stretched to the limit for the past few years. There have been times when there was nothing left in the locker, when all of our assets were being used on one operation.
'At the same time, we have to contend with the very real threat being posed by the Russians. Russia is a country which is under suspicion of committing murder on British streets and it must be assumed that having done it once, they will do it again,' Britain's Telegraph newspaper yesterday quoted the source as saying.
The source added that MI5 was so stretched that some recent counter-terrorist operations against Muslim extremists had used up its entire surveillance resources, meaning other areas of security and intelligence work had inevitably suffered.
Prime Minister Gordon Brown will be meeting Russian President Dmitry Medvedev for the first time at the Group of Eight (G-8) summit this week. But it is not known if Mr Brown will raise his concern over the levels of industrial and military espionage by the Russians.
Such a move would worsen the already cool relations between the two countries which resulted in a row between ex-British leader Tony Blair and former Russian president Vladimir Putin at the G-8 summit last year.
But in an interview in Hokkaido - the venue for this year's G-8 summit - Mr Medvedev showed signs of wanting to end the rift, saying international relations required people to come together.
The Russian intelligence services, the prime suspects behind the murder in London of former KGB agent Alexander Litvinenko, are believed to have a network of some 30 spies operating in Britain.
The agents, whose numbers are equivalent to one in five of the Moscow government officials based in Britain, are known to be monitoring the movements and activities of Russian emigres and opponents of the regime.
But they are also targeting businessmen, MPs and scientists looking to steal commercial and state secrets. Only in the United States, it is understood, are there more Russian agents.
One security source said: 'Russia has a spy culture that predates the communist regime. The Federal Security Bureau is just as active as the KGB was during the Cold War. They have a policy of state-sponsored assassination and they pose a very credible threat. They want to steal our secrets. They have an insatiable appetite for anything to do with arms manufacture and energy.'
In July last year, Britain expelled four Russian diplomats after a request to extradite the prime suspect in the Litvinenko murder case was refused by the Russians.