Treasury Secretary James A. Baker III yesterday said that the United States now favors a major increase in funding for the World Bank to enable it to expand its lending to poor nations and Third World borrowers.

The Reagan administration had been resisting calls for new funding for the bank. But Baker, appearing at a Treasury briefing on the joint World Bank/International Monetary Fund meetings that begin here this weekend, said the need for the capital increase had been demonstrated and "the time is now ripe" to start negotiations for it.

Baker didn't specify how much additional funding -- called a general capital increase -- he would seek. But he said discussions would center on the bank's own estimate of what will be needed.

World Bank President Barber B. Conable Jr. has been pushing for an increase in the range of $40 billion to $80 billion.

Baker said that having increased its disbursements to the major debtor countries -- mostly in Latin America -- by more than 40 percent since 1985, the World Bank is clearly in need of added capital.

"The bank is playing a key role in the implementation of the international debt strategy, and is taking the lead in supporting economic reforms that have been undertaken by developing countries," Baker said.

He added that under Conable's leadership, "the bank is doing an excellent job in promoting the types of investments and types of policy reforms that we think are essential to achieve sustained economic growth in the developing world."

Baker acknowledged that the plan would meet resistance on Capitol Hill, where legislators are concerned about the budget deficit.

The U.S. share of a new capital increase would be 18 percent of the total, but only a small fraction of that would actually have to be "paid in" to the bank -- and thus become a budget expenditure item. Most of the money would be "callable," to be provided to the bank if needed.

For the 1980 increase of $40 billion in the bank's capital, to a present total of $80 billion, the U.S. share was 20 percent, or $8 billion. At that time, the paid-in portion was 10 percent, or $800 million. Baker said yesterday that the annual budget cost has been only $100 million a year.

Conable, it was learned, has given Baker and representatives of other key shareholding countries a blueprint of his plans to use expanded resources over the next few years. Spokesman Frank Vogl said that Conable would outline these plans, which call for larger loans to the major Latin borrowers over the next several years and more help to poverty-stricken countries, in his speech next week to the joint World Bank/IMF meeting.

Vogl also said that Baker's approval of Conable's proposal for increased funding, in principle, came after the bank's management had demonstrated that "the recent reorganization is now complete, and that the bank is in position to take on a greater role."

World Bank expert Richard Feinberg of the Overseas Development Council, on the other hand, suggested in an interview that Baker's surprise announcement may have been prompted to "defuse some of the pressure building up for action to relieve the debt crisis. By agreeing now to a GCI {general capital increase}, Baker draws attention to a responsive and revamped World Bank, and away from recalcitrant commercial banks."

Large debtor nations such as Brazil and Argentina are nonetheless expected to be bitterly critical of the commercial banks, and of the richer nations, at next week's annual meeting.

Baker stressed that plans for a general capital increase for the World Bank are "in no way intended to be a substitute" for continued lending by the commercial banks. The World Bank itself has been sharply critical of commercial banks for failing to boost their loans to 15 major borrowers, as Baker had asked them to do two years ago at the IMF/Bank meeting in Seoul.

Officials said that the paid-in percentage for the general capital increase might be reduced as part of the negotiations among the contributing countries and the bank.

Theoretically, the paid-in share could be zero, but the bank is anxious to have some amount actually paid in. On the strength of its total capital resources, including the callable amounts, the bank can borrow in the world's financial markets. But its total loans outstanding at any time may not exceed the total capital.

Bank officials, led by Conable, have been saying for some time that without a capital increase, the agency's loan limit would be reached in a year or so, and that it is critical for a decision on a capital increase to be made within the next 12 months. There was general support for this view at the economic summit last June.

Baker predicted that if the pattern of the negotiations for the last general capital increase could be taken as a guide, the negotiations could be completed, and the first capital paid in, by September 1988. However, the ODC's Feinberg said that unless the Reagan administration asked for a special supplemental appropriation next year, it is not likely that any American paid-in capital could be transferred to the bank until 1989.

Vogl said that Conable expects that the negotiations can be completed by next June, and that, regardless of whether any capital is paid in next year, approval of the plan would enable the bank to expand its lending operations in fiscal 1989.

Asked about the timing of his announcement, Baker said "we have always said that the United States would be there when the need was there. We think the need is there now. We're convinced we ought to get it going right away."