A. W. Clausen, president of the World Bank and its private-sector subsidiary, the International Finance Corp., said yesterday that development aid agencies such as his own should do more to promote growth through the private sectors in the Third World, accelerating the denationalization process "under certain circumstances."

In a speech to the Institute of Directors in London, Clausen said he was not intending "to press a case against public ownership." But he argued that over the last two to three decades, "the thrust of the development effort has been directed toward public sector activities," rather than to promote private business operations.

"That emphasis, in my view, is misplaced," Clausen said. "Aid agencies ought to be taking, and must take, a more balanced approach." He pointed out that even though the private sector generates almost 75 percent of the gross domestic product of the developing countries, "not nearly enough is known of what the private sector can or cannot do."

Clausen said the bank and the IFC -- which promotes and supports private enterprise in the developing countries -- have "a significant role" in helping governments to create an environment in which private businesses can flourish.

He said the bank and the IFC "see themselves as partners with the private sector in the economic development of the Third World."

The IFC, under the new leadership of executive vice president Sir William Ryrie, is undertaking a five-year expansion program designed to sharply boost its investments in the Third World. In a speech earlier this month in Frankfurt, West Germany, Ryrie noted that leaders in developing countries "are turning their backs on interventionist economic policies in favor of more open policies in their efforts to increase export production, attract savings and investments, and revive their sagging agricultural sectors."

Clausen acknowledged that in recent years, private investment in the Third World has been inhibited by fears of foreign investors for the safety of their funds, as well as host country concern over possible foreign domination and control. But in the wake of the debt crisis, Clausen said some of the attitudes on both sides may be changing.

He noted that to strengthen the incentives for private investors, the bank had earlier proposed a Multilateral Investment Guaranty Agency. The MIGA -- still under consideration by the bank's member-shareholders -- would guarantee against certain risks in Third World investment.

Clausen said the new and expanded IFC approach will be to stress investment in low-income countries, in agriculture-related activities, and energy. The prospective IFC expansion, as well as general endorsement of the idea of stressing private sector rather than public sector development aid, is warmly supported by the Reagan administration.

His reference to potential denationalization was in the context of praise for Great Britain's "successful privatization initiatives in the oil, automobile, telecommunications and public housing sectors." A number of Third World countries "may have something to learn" from that experience, he suggested.

Clausen acknowledged that denationalization would have to be undertaken with caution.