A photograph published yesterday, identified by the wire service that distributed it as showing Soviet defector Oleg Gordievski, was actually of another, unidentified man.

The senior KGB officer whose defection triggered a mass expulsion of alleged Soviet spies from Britain has cooperated with western intelligence services since he worked as a Soviet diplomat in Denmark in the early 1970s, according to sources close to the case.

British officials today revealed that British counterintelligence had been in close touch with the Danish security authorities about the case of the KGB official, Oleg Gordievski, from the time he started passing information to the West. They said that Gordievski had been appointed London station chief of the KGB, the Soviet spy agency, shortly before his defection.

The revelations followed an unusually frank statement last night by Danish Justice Minister Erik Ninn-Hansen, who said that Gordievski had provided Denmark with information of "great importance" during his 10 years as a Soviet diplomat in Copenhagen.

The admission that Gordievski was a double agent casts new light on his defection and the information that he has been able to provide western intelligence officials about the KGB's operations abroad. It also raises the question of whether he decided to defect because he feared that his cover had been blown and that the KGB was on to him.

British officials today formally denied that Gordievski's defection was directly linked to the flight of a top West German counterintelligence official, Hans Joachim Tiedge, to East Germany last month. They said that Gordievski had defected to Britain "several weeks" before the Tiedge affair.

This would appear to place his defection some time in July, or possibly June, as the Soviet Embassy identified him as the third-ranking Soviet diplomat in Britain in the June 1985 issue of the diplomatic list. Officials here did not explain why they waited two months to make the defection public, or why the announcement was made this week.

British officials were extremely cautious in their public comments today on Gordievski's case, arguing that they wanted to make the task of reconstructing his life as a double agent as complicated as possible for the Soviets. But they went out of their way to rebut suggestions that credit for recruiting Gordievski should go to Denmark.

"There is no question of an Anglo-Danish row over this. But any idea that we simply took over a Danish agent is simply not true," a British official said.

The official added that Britain had cooperated with Denmark on the Gordievski case "from the outset." He refused to specify exactly when that was, but it is understood that the KGB official began supplying information soon after he arrived in Denmark on his second tour in 1972.

Gordievski, 46, who joined the KGB in 1962, was stationed in Copenhagen from 1966 to 1970 and from 1972 to 1978, rising to the rank of first secretary in charge of relations with the press, according to Danish officials. He arrived in Britain in 1982.

As a member of the third department of the KGB's directorate for foreign espionage, Gordievski specialized in operations in Scandinavia and Britain, according to British sources. This would explain why any information he provided to western intelligence agencies during the period that he worked in Copenhagen would have been shared with the British.

The Danish justice minister's description of Gordievski's role as a double agent has triggered a political row in Denmark and some irritation in Britain. The minister was accused by his predecessor, Social Democrat Ole Espersen, of damaging western intelligence by saying too much.

British espionage specialists were unanimous today in describing Gordievski as one of the most important intelligence catches by Britain since the war -- a period more noteworthy for the series of defections in the opposite direction.

"This is enormously discouraging to the KGB. It is not just Soviet agents in Britain who have been exposed but also a number in other countries . . . . Gordievski presumably carried with him information about 'sleeper' agents in the other countries he was dealing with," noted Peter Reddaway, an expert on the Soviet Union at the London School of Economics.

Reddaway said he believed that it would take the Kremlin considerable time to rebuild its network of agents in Britain, and he speculated that they might shift their effort to other Western European countries. He said that any Britons now working for the KGB clearly would be at risk of exposure following Gordievski's defection.

On the other hand, the defection means that Britain loses a key source of information inside the Soviet intelligence system. There is no indication of how much damage the alleged spies operating out of the London embassy may have done to British security.

Other intelligence experts said that Britain remained a prime espionage target for the Soviet Union in view of the large numbers of U.S. military personnel stationed here and Britain's key role in the North Atlantic Treaty Organization.

According to British intelligence sources, Gordievski cultivated contacts with the trade union movement, antinuclear and pacifist groups and church organizations during his three-year stay in Britain.

British officials denied reports that Gordievski had defected because of a weakness for western-style living. They described him as intelligent, self-effacing and apparently uninterested in the kind of material rewards he could expect as a defector. Previous Soviet defectors warned that, as an operational KGB officer who went over to the other side, Gordievski will be at risk permanently. He will be obliged to live in a succession of safe houses and to take steps to change his identity.