Amid signs that the nearly 11-month-old strike by British coal miners is crumbling, Prime Minister Margaret Thatcher made it "absolutely clear" today that she would not yield on the crucial point of the dispute. This was despite pledges by the mine workers' union that it would now negotiate without any "preconditions."

In a stormy debate in Parliament, Thatcher said the final decision on whether to shut down money-losing coal pits "must rest with the management of the National Coal Board" that manages Britain's nationalized mines.

Thatcher's staunch defense of her Conservative government's position comes as the strike appears to be moving into a climactic phase, and when the fate of possible negotiations to end the frequently violent strike hangs in the balance.

The leader of the National Union of Mine Workers, Arthur Scargill, has demanded from the outset that Britain's 175 coal pits should be closed only if exhausted or unsafe.

Both sides now appear to be expressing greater willingness to end the dispute, which has cost Britain billions of dollars and created severe hardships for some 110,000 striking miners.

But the government has signaled its determination that a strike called over what are said to be unreasonable economic demands, and pursued by intimidation of those miners who want to work, shall not succeed.

The union leadership, faced with weakening support in the ranks, is trying to salvage what it can from its argument that closing mines entails the enormous social costs of closing down mining villages. It is also trying to win at least some concessions on who determines pit closures after having had its members go without pay for 47 weeks in support of that point.

Early today, Scargill said his leadership was ready to renew negotiations without preconditions, as had been agreed late last week in informal talks with Coal Board members. During those talks, the sketchy outline of a peace plan had also been discussed.

But later today, the Coal Board, in a move that stunned the union, demanded that before negotiations begin the union must put in writing whether it is prepared "to help resolve the problem of dealing with uneconomic capacity in the coal industry."

Union officials had said they were ready to negotiate "everything" and they characterized the demand as an effort to scuttle the talks.

In Parliament, Neil Kinnock, the leader of the opposition Labor Party, said he sensed that the new demand bore the "dirty fingerprints" of Thatcher. The Coal Board, which tends to be more conciliatory than the government, is supposed to be independent of direct government control in the dispute. But it is clear that the government plays a role.

Kinnock asked Thatcher: "Do you really want to encourage negotiations or are you still obsessed with securing humiliation, no matter what the cost or how long it takes?"

For the past few months, as many miners have gone back to work, it has appeared that the strike could ultimately collapse when enough miners drift back to work with no formal settlement -- which could benefit the government.

About 50,000 of Britain's 188,000 miners have worked from the start because the strike was called without national balloting. Since November, another 25,000 or so have gone back to work, according to Coal Board figures.

Mass picketing and picket-line violence have diminished, and Scargill has made fewer strong speeches. Some workers now are even walking through picket lines rather than riding in vans escorted by police.

With the prospect of new negotiations, however, wavering strikers could decide to stick it out.

Thus, government officials acknowledge that new negotiations represent a gamble. This explains the union charge that the government is sabotaging talks in the hopes of winning without having to negotiate.

But Thatcher said there had been seven rounds of negotiations already, that all were fruitless because Scargill has never budged from his "impossible demand" of no pit closures, and that getting it in writing before any new round was essential to having "a clear basis" for new talks.

"I am anxious for there to be a settlement, a clear settlement," she said. But there must be "no fudging."

The Conservative leader in the House of Commons, John Biffen, said the strike was on "its last legs," and Treasury Secretary Nigel Lawson said, "Fortunately, it looks as if it won't be too long now before this damaging strike is over."

Privately, however, officials close to Thatcher were cautioning that it could still be months before the strike is settled. There was no clear indication tonight whether the new talks would go ahead, in light of the new demands.

In another labor dispute that has crippled Britain's main port of Southampton for three months, dock workers today accepted proposals of a mediation service. Virtually all of the container-ship business that supplies much of Britain's shipping revenues has gone elsewhere, stirring fears about the port's future.