Prime Minister James Callaghan is a canny politician who senses when the time for an idea has come. This week he has thrown his considerable weight and growing prestige behind the controversial notion of putting factory workers on corporate boards.

When the first worker-directors take their seats here - perhaps in three or four years since Britons prefer big changes in small steps - the United Kingdom will have joined a group of northern democracies that have introduced similar plans and are distinguished for economic success.

Some form of worker-director legislation is already on the books in West Germany, the Netherlands, Belgium, Norway, Sweden, Denmark and Austria. By no mean coincidence, all enjoy high levels of income, relative industrial peace and minuscule Communist parties.

To be sure, the plan is running into plenty of flak here. Most big businesses see it as diminishing their power and perogatives. The far left is equally unhappy because, as the Communist Morning Star said Wednesday, the scheme "seeks to reconcile the irreconcilable conflict between private profit and labor power."

In the United States, the National Association of Manufacturers would no doubt be as hostile as the Confederation of British industry is to the idea. George Meany would probably dismiss it as has Hugh Scanlon, the former chief of the powerful Engineering Workers.

The ostensible reason for the plan is lofty. The White Paper describing outlines of proposed legislation begins:

"In a democratic society, democracy does not stop at the factory gate or the office door. We spend a large part of our lives at work and invest our skills and energies in industry. There is growing recognition that those of us who do so should be able to participate in discussions which can vitally affect our working lives and our jobs."

That sounds like Shirley Williams, the highly regarded education minister who chaired the Cabinet committee that wrote the paper.

But Callaghan, a strong admirer of West Germany's economic success, is convinced that workers on the board will do more than enhance shop floor dignity.

He knows that in the modern world knowledge is power and power compels responsibility. He sees more responsible union behavior flowing from access to knowledge, from granting Walter Reuther's old demand, "Let's look at the books."

Worker-directors would discuss with their corporate peers plans for investment, the introduction of new (and labor-saving) technology, new acquisitions, the closing down of inefficient plants, profits and profit goals and more.

With a voice in decision-making, it could be harder for unions to strike to preserve obsolescent jobs, to insist on wage increases that would simply force price increases. That, of course, is why traditionalist union leaders like Scanlon and pressumably Meany want no part of it.

But the more sophisticated industrialists here, as well as union leadership as a whole, is convinced benefits will outweigh the costs.

The British government, like most in northern Europe, is now convinced that high employment and stable prices can be achieved only through an incomes policy - the guideposts, guidelines and wage ceilings that Meany has also set his face against. A system of workers on the board, transmitting back to union members an intimate knowledge of a company prospects, is seen as one way of easing the adoption of an incomes policy.

Earlier this year, for example, Sweden's powerful unions agreed to increases of less than 3 percent over the next 18 months despite inflation near 10 percent. The unions took this sharp cut in real income because they and their members have a keen sense of the economic state of affairs. In the same way, the moderate wage bargains struck in West Germany are the envy of industrialists everywhere.

Callaghan's plan - really a tentative proposal - would cover all companies with more than 2,000 workers. If a majority of a firm's workers wanted representation, they could claim up to one third of the board's seats.

To make this less painful for management, Callaghan proposes that British corporations split their present board of directors in two. Workers would sit on a supervisory board guiding general company policy. Day-to-day decision making would rest with a management board, free of worker-directors. This is how the Germans do it, and that, in Callaghan's mind, is an argument.

There will be several years of debate in Parliament and out, before anybody in overalls moves into a panelled board room here, but few here will now disagree with the White Paper's blunt assertion:

"This development is no longer a question of 'if' but 'when and how'."