French President Francois Mitterrand has served blunt notice that there is no way the United States will be able to avoid consideration in next week's North-South economic summit of two Third World demands the Reagan administration has already categorically rejected.

In a letter delivered last week to Mexican President Jose Lopez Portillo and Austrian Chancellor Bruno Kreisky, co-chairmen of the summit to be held in Cancun, Mexico, Mitterrand said he would "put two questions" to each head of state. Do you favor, Mitterrand said he would ask, "global negotiations" among all affected nations to settle outstanding problems, and creation of an energy "affiliate" within the World Bank to finance discovery and production of oil and other fuels in the Third World?

Mitterrand knows the Reagan administration has rejected both ideas. His questions therefore imply, an administration source said, that the administration should reverse its stand or face embarrassment and possible failure of the summit. But top officials said flatly that his warning had changed no minds here.

The administration's current party line for Cancun is contained in President Reagan's Philadelphia speech of yesterday, although officials say they have yet to work out exactly how the various themes in that speech will be presented at the summit. Nor has a decision been reached on what kind of follow-up to Cancun will be sought by the administration.

The Philadelphia speech reiterated the president's view that there should be no new large-scale aid program or transfer of funds from rich to poor countries, and his belief that the poor countries should instead rely more on the private market system to improve their standards of living.

Although it relates to form rather than substance, support of "global negotiations"is a litmus test used in the Third World to distinguish true friends from reluctant supporters. Mohammed Bedjaoui, Algeria's permanent representative to the United Nations, said here yesterday that "at Cancun, the United States has a unique opportunity to break its isolation" from the poor nations by abandoning its opposition to global negotiations.

But a high administration official said that "global negotiations as presently construed would not lead to anything except a sterile and unproductive debate as in the past." The phrase itself has become a buzz-word for transferring discussion of economic problems to the United Nations, where the United States often finds itself outvoted.

As to the energy affiliate, Treasury Undersecretary Beryl Sprinkel said in a telephone interview that "our position hasn't changed; we're not in favor of another bureaucracy. We're in favor of economically viable loans through the World Bank, but we're not interested in subsidizing energy production at any cost."

A possible compromise on the touchier global negotiations question was suggested yesterday by Mahbub ul Haq, director of the World Bank's policy planning department, who is highly regarded by both sides. At a conference arranged by the Overseas Development Council here, ul Haq said there was "a middle ground in which substance could triumph over procedure."

Ul Haq's idea is to continue North-South summits on an annual basis to deal with specific trade, aid, and other issues, and go to the United Nations' committee of the whole only for ratification, not negotiation of agreements. "The poor countries are coming to Cancun in a mood of accommodation and conciliation, not to put President Reagan on a spot," he said.

Reagan administration officials said that ul Haq clearly was floating a trial balloon, but given his Third World connections, they take it seriously. "We will listen to everything," a State Department official said. In terms of considering an "annual Cancun," one policy-maker said: "We haven't crossed that bridge yet."

The question of "global negotiations"--even the phrase--has symbolized the sometimes bitter fight between the rich and the poor, or North and South, nations over aid since the first "oil shock" of 1973. The United States fears that it would be isolated in the United Nations if crucial financial questions were transferred there from the World Bank and the International Monetary Fund, where it effectively can control policy decisions through its veto power.

But the poor nations, with strong support from France and Canada, have contended that "global negotiations" are necessary to rectify an unfair system that deprives them of a fair share of the world's economic wealth. Earlier this month at the Commonwealth heads of government meeting in Australia, Canadian Prime Minister Pierre Trudeau put the argument this way:

"The time is long gone when a small group of countries could think they could sit down in a back room and negotiate for the rest of the world. We have all accepted the United Nations resolution calling for a round of global negotiations, within the universal forum provided by the United Nations, while protecting the competences of the specialized bodies of the U.N. system."