Already cool relations between Britain and the Soviet Union plunged Thursday with the announcement that a senior official in the Soviet secret police had defected here, with a list of 25 alleged Soviet spies. The British Foreign Office announced that the 25 Soviets were being expelled and that Oleg Gordievsky, the London chief of the Soviet KGB, had asked for political asylum.
According to the Foreign Office, Gordievsky had been working for the KGB since 1962. He came to Britain in 1982 with the rank of counselor.
The mass expulsion, including six diplomats, is the largest since 1971 when a Conservative government ordered 105 Soviet officials home. It was expected to provoke a tit-for-tat response from the Soviets.
In counterespionage circles, the crackdown on Soviet spies and the decision by Mr. Gordievisky to defect, were seen as major reversals for Soviet intelligence which has been having great success in West Germany lately.
Peter Reddaway, a Soviet affairs specialist, described the defection of the KGB's London chief with the list of 25 spies as ``a tremendous coup.''
Some of the spying was thought to have been industrial and scientific espionage, but there would certainly have been political espionage as well, Mr. Reddaway said.
But British officials noted that the KGB does not direct agents concerned with military matters, and Gordievsky, despite his seniority, would probably be unaware of their activities.
Still, the defection of Gordievsky has enabled the government to learn the scale and boldness of current Soviet espionage activity in Britain, officials said.
The latest move follows one in last April, when the British foreign secretary, Sir Geoffrey Howe, ordered the expulsion of five Russians on spy charges.
There was concern in some political quarters that the expulsions would destroy British government attempts in past months to improve relations between London and Moscow.
David Owen, leader of Britain's Social Democratic Party and a former Labour government foreign secretary, warned against ``crowing and the use of extravagant language'' in British reactions to the defection of Gordievsky. The affair should be played in a low-key way, he said, and ministers should make it clear that they do not want British-Soviet relations adversely affected.
But immediate reaction in the Soviet Embassy was reported to be cold fury when the defection and expulsions were announced.
Dr. Owen said he expected the Russians to order expulsions of British officials from Moscow. This happened after the April expulsions.
Two years ago Prime Minister Margaret Thatcher attempted to improve relations with Moscow and to develop trade contacts.
But her efforts have been greatly hampered by the discovery of Soviet spying in Britain.
Foreign Office officials said Thursday that they hoped normal relations at the political level could continue, and drew attention to last year's successful visit to London by Soviet Politburo member Mikhail Gorbachev, who shortly afterward became Soviet leader.
But when 105 Russian spies were sent home to Moscow in 1971, the Kremlin's immediate reaction was to accuse Britain of ``spy mania.''
Diplomatic relations between the two countries suffered badly for several years.