Prime Minister Margaret Thatcher tonight ruled out all three options for linking British-ruled, predominantly Protestant Northern Ireland with the Republic of Ireland that were contained in the New Ireland Forum report of last May produced by the four mainstream nationalist political parties of north and south.

"Northern Ireland is part of the United Kingdom because that is the wish of the majority of her citizens," Thatcher said, and change "could only come about with the consent of the people of Northern Ireland." Britain, she said, could not impose a political settlement. "It just would not work," she added.

Although opposition of the British government to the proposals was known and had been stated by ministers, today marked the first time that Thatcher had stated those views publicly.

Irish Prime Minister Garret FitzGerald, a prime force behind the forum, declined to answer directly reporters' questions about Thatcher's attitude toward the proposals.

Expectations had been scaled down recently, but the overtly negative public reaction by Thatcher could hurt him politically at home, some observers here believe.

But at separate press conferences after a two-day summit meeting, both Thatcher and FitzGerald stressed that they had their best, most detailed and most realistic discussions on trying to solve the Northern Ireland problem since the summits began in 1980.

"We are seeing whether we can devise practical proposals which will lead to a fundamental improvement in the situation," Thatcher said.

Both leaders agreed to meet again early next year -- a quickened pace for the normally annual get-togethers. The timing also appears to be aimed at announcing some steps toward improved security or easing of tensions between the Catholic minority and Protestant majority in the north before local elections in May.

Irish and moderate Catholic leaders in the north say that if the sectarian violence that has killed 2,400 people in the past 15 years continues, and the Catholic minority becomes further alienated, then Sinn Fein, the political wing of the outlawed and Irish Republican Army, may become the main Catholic political voice in the north.

Under questioning, Thatcher noted each forum proposal for some new form of government for the north -- a unified Irish state, a confederation, or joint authority by Britain and Ireland -- and emphatically said, "That is out," to each one.

Later FitzGerald said the report also was open to other suggestions and "the fact is that today we reached a very considerable measure of agreement about the problem and we have a shared commitment to try to resolve it. It is clear the prime minister has been coming to grips with what is a very complex and difficult problem."

Both he and Thatcher suggested that at least improvements in security for all the people of Northern Ireland might eventually grow out of more Anglo-Irish cooperation.

Thatcher also said there was "no reason" why FitzGerald "should face any criticism." Thatcher and her Cabinet minister for Northern Ireland, Douglas Hurd, also put emphasis on trying to get Catholic and Protestant parties to resume meetings in the north.

Thatcher mentioned questions of policing, prisons and the judiciary in the north, which Irish officials felt was encouraging because the Catholic minority has complained about all three.

But both leaders spoke only in generalities, said they had made no decisions and offered no assurances of progress. Thatcher said she would not be "cross-examined" by the media and "would not be trapped" into discussing minutiae in which analysts would find "significance that isn't there" on a subject where "one slip of the tongue" can cause grave problems.

A joint communique was similarly bland, although it did commit the leaders to another meeting, to improvements in security and to safeguarding the identities of both the majority and minority communities in the north.

FitzGerald declined to say he was disappointed at Thatcher's attitude toward the forum report. The eight hours of talks, held amid the tightest security and secrecy, produced an agreement for a political dialogue about the security and politics of the north, he said. Whether or not this would evolve into the basis for an enduring settlement, "no one at this stage can predict. Time will have to tell," he said.

Thatcher said there were no rabbits to be pulled out of hats, that if there were an obvious answer it would have been found during the past 15 years. "But we need to try again," she said.

But the two press conferences also revealed other differences. FitzGerald worried about "alienation" of the Catholics in the north, while Thatcher said this was "a word that has crept into the discussion and is not a good one."

While FitzGerald agreed that talks between the political parties of the north, though not fruitful thus far, are important, he said, "That doesn't mean that we two sovereign governments can absolve ourselves" of responsibility.

The two-day summit was to have been held in Dublin. But it was switched to Thatcher's secluded estate outside London at FitzGerald's suggestion because of security considerations after the IRA bomb attempt on Thatcher's life last month.