LONDON, JULY 19 -- The executive board of the National Union of Mineworkers today voted to sue its embattled leader, Arthur Scargill, to recover about $2.5 million it claims he wrongly withheld from the union during its fateful national strike six years ago.

The move is the latest chapter in the long decline of the once powerful union and its controversial leader, a fiery leftist who almost brought Britain to its knees during the year-long strike that directly challenged the authority of Prime Minister Margaret Thatcher's government. Scargill contends the lawsuit is part of a plot by opponents to force him to resign.

Thatcher eventually broke the strike and won the confrontation, greatly enhancing her own stature, while leaving a bitter but unrepentant Scargill to pick up the pieces and the union dramatically weakened. Its bargaining rights were forfeited by the strike, and its membership in the fading industry has plunged from 235,000 in 1984 to less than 50,000 today.

The controversy over the missing funds is another legacy of that labor war. In an attempt to keep union money out of the hands of the courts at a time when the government was seeking to crush the illegal walkout, Scargill set up a series of shadowy organizations to handle strike funds. One of them was the Paris-based International Miners Organization, of which Scargill was president and mineworkers union general secretary Peter Heathfield was a senior official.

The organization collected money from overseas labor groups -- including more than $1.5 million from Soviet mineworkers -- that union officials contend should have gone to the striking miners in Britain. Instead, according to a recent inquiry by Gavin Lightman, a respected London lawyer hired by the union's national executive committee, the funds remained with the international organization under the control of Scargill and Heathfield.

Lightman's 253-page report cleared Scargill and Heathfield of the allegation that triggered the inquiry -- that the two used money that Libya donated to the union during the strike to pay off home loans. But the report charged that Scargill had channeled funds to the international organization that should have gone to the union and used it for personal projects. It alleged, for example, that he borrowed $150,000 from the organization five months after the strike to buy a new house and lent money to Heathfield and paid for house repairs, without union knowledge.

Lightman said that in trying to trace the missing funds, he received no cooperation from the organization, which he described as "practically impenetrable." As for Scargill, Lightman added, "I regret it has been my strong impression that Mr. Scargill's story on a number of points has changed as it suits him throughout the conduct of this inquiry."

Scargill has insisted the money he deposited with the international organization was specifically designated for that group by contributors. Last week, one Soviet mineworkers' official supported Scargill's claim, while this week another who was active at the time of the donation flatly denied it.

In the past, the 14-member executive has backed Scargill in his running battles with union moderates seeking to dethrone him. But at a meeting in the northern mining city of Sheffield today it unanimously voted to follow Lightman's recommendation and sue to recover the funds. Tonight its lawyers obtained a court order freezing Scargill's and Heathfield's funds pending a full court hearing.

Scargill described the lawsuit as unnecessary and unfair. "It just seems to me to be completely crackers to spend money {on legal fees} which ought to be spent for the benefit of miners," he said at a press conference. "The most sensible way to resolve this problem would be to sit down and talk about it."

Scargill mocked suggestions that he resign. "What do you want me to do, run a fish shop?" he asked.

Leaders of the opposition Labor Party, who view Scargill as a political liability, expressed quiet approval of the lawsuit. "I think {the executive committee} is now being seen to be doing its duty," said Kim Howells, a former union official who is a Labor member of Parliament. "I hope now that the whole sordid mess will be cleared up by this action."