Pakistan announced yesterday that France has backed out of an agreement to provide it with a nuclear reprocessing plant that would produce plutonium suitable for use in manufacturing an atomic bomb.

The announcement, made by Pakistani ruler Gen. Mohammed Zia al Haq at a press conference in Rawalpindi, represents a major triumph for the Carter administration's nuclear non-proliferation policy.

The administration is hoping for an additional piece of good news in the next couple of weeks in the form of a decision by South Africa to sign the nuclear nonproliferation treaty.

France's decision not to go through with the Pakistani deal is also expected to bring new pressure on West Germany, which has sold a pilot nuclear reprocessing facility to Brazil.

West Germany is now the only country still planning to go ahead with plans to provide a nuclear reprocessing facility to a Third World nation.

Officials in Bonn insist they have no intention of cancelling the Brazilian sale, but U.S. sources feel Germany may in the end decide to proceed "as slowly as possible" in providing the reprocessing equipment.

Gen. Zia in announcing the French decision, said he had been informed that France was backing out of the agreement in a "very polite" letter from President Valery Giscard d'Estaing.

"In military terms, we would say that although it was full of sentiment, it was a lemon," Zia said.

In Paris, the government confirmed last night that Giscard had sent a letter to Zia concerning the deal but suggested that the French president had once again offered to amend the original deal to provide Pakistan with a nuclear co-processing plant.

Both types of plant take the spent fuel from nuclear power stations and produce fuel that can be used again. But while a reprocessing plant yields plutonium suitable for weapons, a co-processing facility would not separate the plutonium from the uranium. This mix could not be used in fashioning a nuclear bomb.

Pakistan, while insisting that it has no intention of developing a nuclear weapons capability, has adamantly rejected France's efforts to substitute the co-processing facility and has insisted that Paris honor the contract signed in 1976.

This insistence has led many observers to conclude that Pakistan's primary motivation is to gain the capability to join subcontinent rival India - which exploded an underground atomic device in 1974 - as a nuclear power.

These suspicions are buttressed by the fact that few experts in nuclear energy accept Pakistan's arguments that it needs the facility to supply a network of nuclear power plants.

At present, Pakistan has only one atomic power plant in operation, and one other plant in the works. It seems unlikely to have the dozen or more required to make a reprocessing facility economically viable until at least the middle of the 21st century.

The French decision not to go through with the sale to Pakistan marks a final turnabout from the course undertaken by the government of former Prime Minister Jacques Chirac, which also had negotiated to sell a reprocessing facility to South Korea. U.S. pressure on the Korean government was a major factor in aborting and deal.

France and West Germany later announced that while they intended to honor their contracts with Pakistan and Brazil respectively, they would not sell additional reprocessing facilities to Third World nations.

French officials say that since Chirac's departure, France has taken a much more serious view of the non-proliferation implications of the spread of nuclear technology. Giscard has been trying to gracefully back out of the Pakistani deal for some time.

But the Carter administration, with its emphasis on nuclear nonproliferation, certainly deserves "considerable credit for the whole major change that has taken place in world sensitivity" to this issue, one observer noted yesterday.

The administration official who has led the nonproliferation effort, Deputy Undersecretary of State for Security Assistance Joseph Nye, took pains to emphasize yesterday that the reprocessing deal is 'a matter between France and Pakistan."

But both the United States and Canada have brought pressure on Pakistan in an effort to persuade it to cancel the deal.

The Carter administration last October informally cut off all but food aid to Pakistan, and has not signed any new aid agreements with it since that time.

Canada, which built Pakistan's only operating power plant outside Karachi, also announced in late 1976 that it was ending its nuclear cooperation with Islamabad and would not supply uranium fuel for the facility. As a result, the plant has been operating at less than 70 per cent of capacity.

Canadian officials hailed news of the French decision yesterday.

The Carter administration's hopes that South Africa will shortly agree to sign the nuclear nonproliferation treaty grow out of a visit to Pretoria by top U.S. nuclear negotiator Gerard Smith earlier this summer.

Smith, according to informed sources, outlined to the Pretoria government a proposed method for international inspection of uranium enrichment facilities that could be used in safeguarding South Africa's pilot enrichment plant at Valindaba.

South Africa has long expressed concern that subjecting the Valindaba plant to inspection would result in piracy of its secret uranium enrichment process.

The United States, in an effort to persuade Pretoria to sign the treaty, has been holding up a shipment of highly enriched fuel needed to operate a South African research reactor. South Africa also is seeking U.S. fuel for two nuclear power reactors scheduled to go into operation in the early 1980s.