Deputy Treasury Secretary R. T. McNamar and Assistant Secretary Marc Leland said yesterday that the World Bank could not realistically expect to expand its role in this decade, and instead should help the private sector attain a greater influence in boosting economic growth of the Third World.

During a day-long conference at the Brookings Institution here on the future of the World Bank, McNamar delivered a major policy address in which he called for new approaches by the bank in the 1980s geared to lower government budgets and a larger role for private corporations.

He rejected as "analytically inadequate" the criticism that U.S.-subsidized assistance to Third World countries is too low. "We cannot expect larger and larger replenishments for concessional financing. The political and economic realities will not permit it," he said.

McNamar's address, in effect, previewed a Treasury report due next month that will recommend a reduction in the role of the multilateral lending institutions, and--notably--a sharp cut in the money appropriated for the International Development Association, the soft-loan arm of the World Bank.

Leland warned that, with limited resources available to it, the bank in the future will "have to make hard judgments" on which countries should be aided and how the recipients should use the money.

If the Reagan administration tells Congress "that it's going to be 'business as usual'--well, first of all we won't do it, and, second, we won't win," Leland said.

The argument over the role of the bank in an era of high costs was joined at the beginning of the conference when former Treasury secretary George Shultz took sharp issue with a plea by former Bank president Robert S. McNamara that the bank be allowed to continue to expand, especially its subsidized loans to the poorest countries.

McNamara said that "the bank can play a major role--contributing both intellectually and financially"--in helping the Third World during the coming decade. "As a nation will we be wise enough to provide the bank the political and financial support it needs?" he asked. "We are not doing so now. I hope we will be in the future."

As World Bank President A. W. Clausen silently took notes, Shultz criticized the bank's image and said it must avoid being typed as too soft in its lending policy. Shultz, now president of the construction giant, Bechtel Group Inc., had been instrumental prior to the 1980 election in winning bipartisan support for Clausen as McNamara's successor.

In a wind-up speech to the conference of about 150, Clausen, while agreeing with Shultz that "the private sector ought to be more involved in the development sector," supported McNamara's suggestion that the bank's capital could be increased at minimal cost by limiting the actual paid-in portions of a country's subscription.

And Clausen called on the United States "to live up to its international agreements" with respect to its commitments to the IDA. He conceded, however, that in the future the IDA may have to charge interest--there is now only a service fee--and that the 50-year maturity on its loans may have to be shortened.

Clausen also revealed that "today's volatile market conditions are forcing us to consider whether we should introduce a degree of variability into our lending rates." World Bank loans now are made at fixed rates; the current interest is 11.6 percent. To continue loans at fixed rates in the present turbulent markets may be a problem, Clausen admitted.

Although "the World Bank is still a welcome borrower," because of the present uncertainties "the market is taking an acute look at the bank to see if it is a quality investment instrument," investment banker James D. Wolfensohn observed.

Wolfensohn noted that the bank has had to pay anywhere from 130 to 160 basis points recently over comparable Treasury issues, which he said was an "unprecedented spread," increasing the bank's borrowing costs. As one temporary fix, the bank announced on Wednesday that it was imposing a l 1/2 per cent fee on all new loans.

Although Clausen made it clear that the bank was ready to adjust to new realities, Shultz pointedly was espousing a more dramatic reorientation of the bank's aims and scope, which would emphasize creation, rather than transfer, of wealth.

"We have to look on the 1980s in a different way," Shultz said. "The greatest mistake would be to expand the bank's capacity to dip into the world's savings for concessional aid. The bank's image of the past few years is that it is helping countries to avoid facing up to realities."

The pattern of bank activity in the McNamara regime was strongly defended by Henry H. Fowler, Treasury secretary under President Johnson, and by Henry Owen, former aide to President Carter for economic summits.