The archbishop of Canterbury issued a stark warning today to the government of Prime Minister Margaret Thatcher that the violence of an unsettled coal miners' strike and the despair of more than 3 million unemployed threaten to transform British political life from consensus to confrontation.

"We need leadership in our national life which will unite and not divide the nation," said Archbishop Robert Runcie, who is the spiritual leader of the Church of England.

The archbishop quickly added that such an assessment was "not an attack on the government." There were people in all parties, he said, who sought the middle ground for agreement, especially in the bitter, almost eight-month-long coal strike.

But the main thrust of Runcie's remarks, in an interview published today in The Times of London, amounted to a powerful challenge to the economic and industrial policies of Thatcher's Conservative government. The archbishop sought to show understanding of the possible benefits that could grow out of Thatcher's tough stance on making Britain more competitive economically. But he raised grave concerns about the social costs and asked, as he put it, "How long can we wait for jam tomorrow?"

Runcie's comments, which came just one day before the Conservative Party opens its annual convention and which drew immediate fire from party spokesmen, reflect the growing willingness of Britain's Anglican bishops to join the national battle over economic policy and the miners' strike.

"We live in a society in which the majority are better off, but nevertheless, there is growing poverty and despair and a sense of powerlessness . . . , and that is why I think people will say we can't allow this state of affairs to continue," he said.

The recovery of economic growth and national pride were worthy goals, he said. "But if the human consequences of such aims mean unemployment on an unprecedented scale, poverty, despair about the future of some communities . . . , then the objectives must be called into question."

Runcie called the picket-line violence between miners and police "part of a national problem. In a society where there is felt to be unfairness or in a society where things matter more than people . . . , it comes out in this awful cancer of violence."

He warned that the rhetoric of confrontation that has characterized the dispute between the government and the mine union contributes to the violence. Without saying which side he was talking about, he said: "The cheap imputation of the worst possible motives; treating people as scum in speech -- all this pumping vituperation into the atmosphere has a deep effect on the possibilities of physical violence."

Unless a middle ground is found, Runcie warned, "the bitterness and anger will spread. Divisions will take generations to heal. I think there is a serious danger that there will be a loss of confidence in the international world that will be bad for our prosperity. And there is a danger there will be an increased authoritarian kind of government either from the right or left."

On Saturday, the bishop of Birmingham also spoke out against government economic policies. Last month, the bishop of Durham, the Rev. David Jenkin, fired the first salvo from the pulpit aimed at the government. Both bishops are from coal mining regions.

Today, however, Durham broadened his attack to include the controversial mine union leader, Arthur Scargill.

Durham, in a continuing exchange of letters with British Energy Secretary Peter Walker, condemned Scargill's "refusal to organize a ballot" of all miners about whether they wanted a strike "and his readiness to organize intimidation" of miners who want to return to work.

He attacked Scargill's "apparent attachment to a Stalinist-type of Marxism, which is thoroughly discredited, both in theory and practice. I do not believe he is as clever as Lenin," Jenkin wrote of Scargill, "and I do not believe that the working classes of this country are ripe for a revolution organized by a Bolshevik-type minority."

But Jenkin warned that the "Scargill phenomenon" would not have come about if many working people "were not feeling themselves pushed into helplessness and hopelessness, and, above all, that they were being ignored."

Conservative spokesman John Gummer called on Runcie to "seek out the truth," and the truth, he said, was that the only way to conquer unemployment was to produce goods that people want and can pay for. That, he said, meant becoming an efficient and competitive economy.