London — A new development has emerged here in the long impasse over Northern Ireland that adds to the pressure on Prime Minister Margaret Thatcher to shift tactics, if not basic strategy.
It could also encourage the Provisional wing of the illegal Irish Republican Army (IRA) to hang on in its political struggle to push the British out of Northern Ireland and set up a "Democratic Socialist Republic" in all of Ireland, north and south.
The development is the first faint sign that broad bipartisan support for the Thatcher tough line against any concessions to IRA hunger strikers may be ending.
The British political right still seems to be firmly behind Mrs. Thatcher and her strategy of no political status for striking prisoners, exploratory talks with Dublin on easing border tensions, and perhaps longer-range plans and overall support for Ulster's embattled 1 million Protestants.
But the British political left has begun throwing out new ideas to settle the current standoff, leading observers to suggest that Northern Ireland may become more and more an issue of political controversy here.
The first sign came when former Labour Cabinet minister Tony Benn launched a carefully calculated statement in a rare appearance on British Broadcasting Corporation radio. British troops must be withdrawn, he said, and replaced by a United Nations peacekeeping force.
Mr. Benn is a leading left-winger, the most prominent in the far-left's current campaign to take over control of the Labour Party itself. In the last 12 months, the campaign has been spectacularly successful, driving many moderates out and leading to the creation of the new Social Democratic Party here.
He is now challenging Denis Healey for the post of deputy leader of the party and seems to have decided to use Northern Ireland as part of his challenge. This, in turn, will lead to more publicity in Britain, will widen the split in the Labour Party, and will be echoed by Republican supporters in Ireland itself.
His idea is that the partition of Ireland 60 years ago was "a crime against the Irish people." They must now be allowed to settle their own affairs: A UN force would keep the peace while they were deciding their future.
Britain, of course, would veto any move toward such a force in the Security Council and would lobby hard in the General Assembly. Northern Ireland's Protestants would also object. But the merits of the proposal mean less at this stage than the fact they are being made.
The next sign of movement on the left came from former Foreign Secretary David Owen, a founding member and parliamentary leader of the Social Democrats.
He told a party meeting in Blackpool that Britain must acknowledge it cannot solve the Irish question alone. Instead, it should convene, with Dublin, a working party of all members of the European Community.
This, he believed, would signal "political will" for solving the crisis.
While he was making his proposal, the Liberal Party was being less controversial but was also looking for new initiatives. Spokesman Stephen Ross urged an all-party group of members of Parliament to get together with figures from all the major parties in Ulster.
The joint group should then visit the Maze Prison outside Belfast, interview staff and prisoners, and look for new solutions to the prison issue itself.
Meanwhile, Mrs. Thatcher shows no signs of yielding on the IRA's central demand of political status for prisoners in the Maze.
She sees several glimmers of support for her position -- at a time when any glimmer, no matter how slight, is more than welcome.
An opposition Labour Party study group that visited the Maze in March has supported the government assertion that conditions in the Maze are actually "first-class" for prisoners who obey the rules.
Some reports from Belfast say that the next hunger striker expected to die, Raymond McCreesh, may not be so determined as Bobby Sands and Francis Hughes before him.
Serving 14 years for attempted murder and other offenses, McCreesh is portrayed as a man who just might be willing to compromise before too much longer. As of May 17, he had been without food for 58 days.
There are also signs that public opinion abroad is more aware of the crimes committed by men like Sands, Hughes, and McCreesh, and more skeptical of IRA claims of political justification.
Labour leader Michael Foot still supports the Thatcher policy, although he has agreed to the need to review official Labour policy when a study group reports to the annual conference of the party later this year.
Mrs. Thatcher still pins hopes to the joint studies under way since last December between London and Dublin. The talks are aimed at finding ways to ease tensions along the border between the Irish Republic and the north. One plan being studied is a new Anglo-Irish council, to which both governments would appoint memb ers of Parliament.