High-level opposition had developed within the Reagan administration to building neutron missile warheads and artillery shells, putting in doubt a once-certain production decision by the new president, according to administration sources.

One source for this opposition: several new, high-ranking State and Defense department officials who have been impressed by arguments from European NATO allies.

These NATO officials have said that a change in the Carter policy, which called for deferring neutron production, would reopen the political, antinuclear debate in Europe and endanger the agreed-upon alliance decision to go ahead with more powerful medium-range nuclear missiles.

There is also an Energy Department concern that not enough of the nuclear material tritium is available to support a neutron weapons building program without hurting other nuclear warhead projects.

The neutron question will be discussed, but probably not settled, at an interagency meeting scheduled this week at the Pentagon.

Although Defense Secretary Caspar W. Weinberger last month said he favored going ahead with the new generation of short-range nuclear weapons, echoing statements made during the campaign last year by President Reagan, these new foreign policy and nuclear material concerns have been enough to hold up and perhaps kill a production decision.

The major influence to drop immediate production of the neutron weapons, sources said, is concern voiced by leaders of the NATO countries last month following Weinberger's statements.

British and West German officials have told Reagan officials that the Europeans would not accept neutron weapons, primarily because they are designed to be used on their soil.

A decision to build the weapon would harm the NATO plan adopted in December 1979 to put more powerful, medium-range nuclear Pershing II and ground-launched cruise missiles in Western Europe.

Under a 1978 decision by former president Carter, production has begun on about 350 low-yield 56-mile-range lance nuclear warheads, which could be converted to neutron with insertion of a device made primarily with radioactive tritium.

A new version of the 8-inch, 20-mile-range nuclear artillery shell, which also could be converted to neutron with the tritium device, is now in final engineering development.

Proponents of neutron weapons on Capitol Hill had tried to get the Carter and Reagan administrations to build the tritum inserts and store them in the United States, shipping them to Europe for mating with the Lance warheads and artillery shells only when there was a military crisis.

The shortage of tritium is being cited as another reason for not going ahead with immediate production of the neutron weapons or the insert devices. The materials shortage stems from competing demands of the major strategic nuclear weapons building program that is already under way and others planned by the Reagan administration.

The United States has embarked on its biggest nuclear weapons program in history with new warheads for its Minuteman III missiles now being produced, new Trident warheads for sub-launched missiles, new air-launched cruise missiles, strategic and tactical bombs, with the Lance and artillery shells coming along. Also waiting to be put into production are warheads for the Pershing IIs and ground-launched cruise missiles.

The Army has been pushing for a quick Reagan administration decision to reverse the Carter approach and go ahead with neutron production. Army officers have told Congress that if the order to begin building were made before mid-1981, there would be no impact created by the Carter delay in Lance neutron warhead production.

If the go-ahead were held up two years, it would take an additional two years to build the tritium inserts, they say.

What is worse from an Army point of view is that the low-yield, not-converted-to-neutron Lance warheads and artillery shells have only about one-third the nuclear power of weapons now deployed.

If the delay in building the tritium inserts looks permanent, Army officers claim they will need additional weapons to carry out the mission assigned to the nuclear warheads and shells.

Neutron weapons, in effect, are very low-yield hydrogen bombs which have radiation as their main killing mechanism, rather than heat and blast as do normal nuclear weapons.

The military services wanted the neutron devices because they would create less collateral damage on the European battlefield. Thus, they would be more likely to be used in the event of a Soviet invasion than larger yield weapons, these officials argued, and thus were more effective deterrents to the Soviets.