The Russian Federal Security Service - Federal'naya sluzhba bezopasnosti, or FSB - still numbering an estimated 75,000 people - covers some of the areas of the British "Security Service" MI5, which is responsible for counter-intelligence and counter-terrorism, and the Secret Intelligence Service, SIS, which spies abroad and is often known by its historic title, MI6.
The Security Service, headed by Stephen Lander, who took over from Stella Rimington just after Easter is based at Thames House, on Millbank in London: SIS, headed by David Spedding, is based across the Thames, at Vauxhall Cross. Each of the British services numbers about 2,000 people, although the numbers are misleading as they also use many agents and informers who are not full-time employees.
Some of the operations undertaken by the British SIS also fall under the purview of Russian military intelligence - GRU, the Main Intelligence Directorate, which has not diminished since the demise of the Soviet Union.
Although the British Intelligence services shared a common origin, they were soon split and developed a different ethos and even, in some cases, a mutual hostility. Whereas SIS maintained a certain glamour through its association with the Foreign Office, MI5's modus operandi is often portrayed as rather mundane, police-style work. MI5 has no executive authority: if they catch anyone spying they report it to Special Branch.
Both services were founded in March 1909 as the Secret Service Bureau under the leadership of Captain Vernon Kell and Captain Mansfield Cumming (known as "C" - the origin of Ian Fleming's "M" in the James Bond spy-thrillers). In October 1909 functions were divided, Kell took responsibility for counter-espionage within the British Isles and "C" for gathering intelligence overseas.
The Russian security service is the heir of the Soviet "Committee for State Security" - the KGB. At its height in the mid-1980s the KGB ran a network of some 400,000 agents in Russia and an army of 200,000 elite troops, including border guards. However, the First Chief Directorate (Foreign Intelligence) - the equivalent of SIS - numbered just 12,000. Immediately after the break-up of the Soviet Union at the end of 1991 the service was split up. The covert elements of the service were split into intelligence and counter-intelligence, mirroring its British counterpart. In March 1995 it was reunited under Colonel General - now Army General - Mikhail Barsukov, a Yeltsin ally.
Barsukov was held responsible for the disastrous handling of the hostage crisis at Budyennovsk last year, and security experts yesterday said the Russian demand for the expulsion of nine British diplomats might be an attempt to restore the FSB's reputation. Significantly, pressure for hard-line action came from the FSB and not from the Russian Foreign Ministry.
The reputation of an intelligence service is as important as its performance, and the British services have been well-regarded of late. Their biggest embarrassment occurred in the 1960s when members of the Cambridge spy ring recruited in the 1930s - Kim Philby, Anthony Burgess and Donald Maclean - were revealed as highly placed KGB agents. Sir Anthony Blunt and Roger Hollis followed.
In contrast to the secretive nature of the British and Russian intelligence services, the US Central Intelligence Agency (CIA) maintains a relatively high profile. Headed by John Deutch, a former deputy defence secretary appointed by President Clinton a year ago, it has 28 separate intelligence bodies covering everything from political and economic to purely military intelligence, where it sometimes clashes with the the Defense Intelligence Agency (DIA), much like its British and Russian equivalents.
The total US intelligence budget is $28bn (pounds 18bn) but the exact amount is secret. Its biggest success was undoubtedly the overthrow of communism: its biggest embarrassment was the Bay of Pigs fiasco in 1961, when the CIA masterminded the landing of a small army of Cuban exiles on the south Cuban coast. The CIA's reputation has been damaged by two recent scandals.
In 1994 it was discovered that veteran CIA officer Aldrich Ames had been selling secrets to the former Soviet Union, and last year it emerged that a murderous Guatemalan colonel had been on the CIA payroll.