London — A donnish, white-haired man with his back to the wall has opened a new and significant chapter in British politics. At stake: Can Labour leader Michael Foot restore his party's credibility as Her Majesty's opposition by challenging and investigating the Troskyite, hard-left group within the party called Militant Tendency.
The group is made up of thousands of young, highly educated extreme leftists who demand that all major companies and insurance houses be nationalized, that Britain give up its nuclear weapons, and that it leave NATO and the European Community.
It is the open challenge to Labour's traditional leaders by groups such as the Militant Tendency (who support Labour left-winger Tony Benn) that has forced moderates to leave Labour and join the fast-growing and fast-moving Social Democratic Party. More moderates are leaving Labour every week.
Mr. Foot's own status as party leader has suffered. So has Labour's appeal to the country as a whole.
Now he has at last decided to yield to pressure from the right of his own party - people like former Prime Minister James Callaghan, home affairs spokesman Roy Hattersley, shadow chancellor Peter Shore, deputy leader Denis Healey - and challenge the hard left frontally.
First he persuaded Labour's national executive to withhold endorsement from left-winger Peter Tatchell as candidate for Parliament from the working class area of Bermondsey in London's borough of Southwark.
Now Foot has won a cliffhanger vote in the executive (10 to 9) to set up an inquiry into the most visible of Labour's hard-left factions, the Militant Tendency.
If he succeeds in banning the group and its weekly Marxist newspaper, he will have gone a long way toward restoring Labour credibility - at a time when the Conservative Party government is highly unpopular because 3 million people are out of work and recession continues.
Foot will also have dealt a blow at the Social Democrats by answering one of its main criticisms of Labour.
''If he carries this through,'' one of Britain's leading political analysts said in an interview with this newspaper, ''it will be significant, indeed. It is reminiscent of Hugh Gaitskell vowing in 1960 to fight, fight, fight, and fight again to oppose unilateral disarmament. Gaitskell eventually won.
''But today, many questions remain. It is too early to tell which way this battle will go.''
The analyst, London Times columnist Geoffrey Smith, saw Mr. Foot's stand as showing the strength retained by the men on his right - Callaghan, Healey, Hattersley, Shore.
Foot now confronts those much further to the left than himself, the forces led by Tony Benn, who has vowed to oppose with all his strength what he calls the ''thought police'' of the Foot supporters.
Militant Tendency has sounded the bugles of defiance. Peter Taaffe, editor of the militant weekly, claims he has 5,000 supporters in 400 Labour constituencies ready to launch an unprecedented grass-roots struggle to stay inside the party.
So the battle lines are drawn. For the moment, Labour's civil war will intensify.
Mr. Foot's critics say he may fail to carry through the investigation. The probe is to look into Militant Tendency's finances, full-time staff, organization, and connections abroad. Already five supporters of the Militant Tendency have been endorsed as Labour parliamentary candidates in the next election. One more has been selected but not yet endorsed by the national executive.
Mr. Foot has clearly chosen the group as a symbol of the hard left.
Moderates like Shirley Williams, David Owen, and Roy Jenkins, all-well known former Labour Cabinet ministers, formed the Social Democrats charging that Labour was no longer run democratically.
Mrs. Williams has said that if Labour moved back toward the center, the Social Democrats could be squeezed out of existence.
Mr. Foot's new challenge to the left is the start of a move inside Labour to do just that.