The Reagan administration is nearing a decision to proceed with the long-delayed sale of helicopter spare parts to the military regime in Guatemala, although the move will trigger more congressional opposition against U.S. military aid programs in Central America.

U.S. officials privately denied yesterday that a final decision has been made. But they acknowledged that the administration is leaning toward approval of the $2 million sale, and that briefings given to congressional staff members this week were intended to signal those intentions.

On Tuesday, Stephen W. Bosworth, deputy assistant secretary of state for inter-American affairs, told staff members of the Senate Foreign Relations Committee and the House Foreign Affairs Committee that the administration believes the sale is necessary to protect Guatemala's military ruler, Gen. Efrain Rios Montt, against a coup that would make even more difficult an improvement in human rights and a return to democracy in that country.

Sources at the closed meetings said Bosworth disputed arguments that Rios Montt's record does not merit U.S. military aid. They said he also disputed contentions that the administration had promised to obtain informal congressional consent for the sale. He also refused to give any commitment that the administration will delay a decision until Congress returns Nov. 29.

Because the proposed transaction involves a cash sale of equipment not designated specifically as military, the administration can give it a green light without formal congressional approval.

However, Rep. Michael D. Barnes (D-Md.), chairman of the House inter-American affairs subcommittee, expressed an attitude that is widely shared on Capitol Hill when he said: "If they do go ahead with this sale, it will be a breach of faith with Congress. We had a clear understanding with the administration that there would be no change in our military relationship with the Guatemalan military without congressional approval."

Barnes and other congressional sources predicted that, if the sale proceeds, the administration will face increasingly stiffer opposition to its requests for military aid for Central America.

They warned that the opposition is likely to apply not only to $250,000 in training funds for Guatemala sought in the administration's pending appropriations request for fiscal 1983, but also to the significantly larger amounts it wants for El Salvador, where support for the government's fight against leftist guerrillas is the linchpin of U.S. policy in the region.

Hostility to the sale is not confined to Congress. The U.S. Catholic Bishops Conference yesterday disputed administration claims that human rights abuses in Guatemala are declining and reaffirmed its opposition to military aid.

Guatemala has been seeking for years to buy spare parts for helicopters used to fight a growing guerrilla insurgency. And the administration, fearing the threat of a leftist takeover in Central America's largest country, has been seeking ways to strengthen its ties with Rios Montt, who took power following a coup earlier this year.

Until recently, U.S. officials, while saying they believe Rios Montt wants to control rights abuses, privately conceded that the situation there had not improved sufficiently to justify a resumption of U.S. military aid, broken off in 1977.

However, the sources said that Bosworth, in his briefings, argued that the sale would strengthen Rios Montt's hand against hard-line rightists in the Guatemalan military and serve as an incentive toward further reform. If he cannot get help from the United States, Bosworth reportedly warned, Rios Montt is in danger of overthrow by the most intransigent elements in the Guatemalan military.