London — On lonely Ascension Island in the South Atlantic, in picturesque Cornwall, on Cyprus and in Hong Kong, British agents listen in to Soviet and Warsaw Pact satellite signals and other electronic intelligence.
Their work is part of NATO's total electronic surveillance effort. The raw material reportedly is fed into National Security Agency headquarters in Washington for detailed analysis.
Both the United States and British governments want to avoid disruption to the flow of such data, especially since a language specialist at the British Government Communications Headquarters (GCHQ), Geoffrey Prime, was convicted of spying for the Soviets in 1982.
But latest British moves to safeguard the flow have touched off conflict here. The US is watching closely.
The British agents belong to six civil-servant unions - unlike agents in the intelligence agencies M.I.5 and M.I.6, who are strictly nonunion.
Unions first said their members had conducted strikes and ''go slows,'' but that security was unaffected. Then on Feb. 1 Bill McCall, a leading civil servant union official, acknowledged that the unions had interferred with security in 1981 in an effort to secure higher pay.
The government is emphatic that security was impaired because data collection was interrupted. Last week it announced the right of the agents to belong to any union was ended, invoking legislation passed by the previous Labour government. The move affects some 7,000 to 10,000 unionists in Britain and around the world.
Each employee is being asked to sign a document promising to leave his or her union. Those who do so will receive receive (STR)1,000 ($1,400) each and have the right to join a ''staff association,'' but they will not have the right to strike. Nonsigners will be transferred to other jobs.
Criticism was immediate and sharp. The Labour Party and the trade-union movement said union membership was a basic right that ought not to be removed.
On Jan. 31 - after four days of controversy - Foreign Secretary Geoffrey Howe replied by citing eight cases of strikes, walkouts, and ''go slows'' since February 1979.
The ailing British union movement has a new issue to try to use against the government. Union power and membership have been declining in recent years. Moreover, security issues are difficult for the public to evaluate, since intelligence collection is top-secret.
It appears that the Thatcher government was surprised at the amount of opposition to the move. To the government, it is clear that unions - six civil servant unions in this case - can, by striking and working-to-rule, impair the collection of raw intelligence.
Sir Geoffrey did not give details of the eight cases, but he said they showed union action for higher pay and better conditions during unrest in Iran, Middle East fighting, the Soviet invasion of Afghanistan, the Polish crisis, and the Argentine surrender on the Falklands.
''On occasion, over 25 percent of GCHQ staff were involved and the degree of disruption could have had serious consequences for national security,'' he said.
Union leaders responded Jan. 31 that the actions were minor. Several took only a single day. Some were in areas far removed from the crises cited. Unions had taken care to see that no vital breaches of security took place. Prior notice had been given each time.
Late that same day Mrs. Thatcher fired back. On the floor of the House of Commons she defended suspension of union membership by saying unions had been deliberately selective in choosing areas at GCHQ where they could do the most damage. For the first time she estimated time lost by union action:
''If you lose 10,000 working days in an agency concerned with continual surveillance,'' she said, ''you cannot know what information you have lost, or what its relevance may be.''
The unions met with Mrs. Thatcher Feb. 1. They offered some of the most far-reaching changes ever proposed in Britain, including a guarantee there would be no disruption again at GCHQ. Mrs. Thatcher stood firm. She insisted that union membership would be removed.
The leader of a large white-collar union, Clive Jenkins, told American reporters Feb. 2 that Mrs. Thatcher took such action because of another issue altogether. Under US pressure, the government is introducing lie detectors for some intelligence workers following the Geoffrey Prime case. Mr. Jenkins said the government wanted to avoid a situation in which union members, objecting to the lie detectors, took their case to an industrial tribunal and either demanded large compensation or began discussing their classified work in public.
The government denies any such intention.
Mrs. Thatcher, who has a large majority in Parliament to fend off opposition moves, also denies US pressure. So, for the record, does the US Embassy in London. But it is known the US has been worried in the past by the union actions.