President Carter will soon have to make another controversial production decision on neutron weapons, informed administration sources said yesterday.

But on Capitol Hill, some legislators, including Sen. Mark O. Hatfield (R-Ore.), who last year led congressional opposition to the new generation of tactical nuclear weapons, are fearful that the new decision may be classified.

All this had occurred because administration officials have been unable to resolve their internal differences over how to implement the president's April 7 announcement that he was deferring immediate production of neutron Lance missile warheads and 8-inch artillery shells.

In making that decision, Carter ordered the Defense Department to go ahead with "modernization" of the Lance missile warhead and 8-inch shell, "leaving open the option of installing the enhanced-radiation [neurtron] elements."

At issue is just what "modernization" should entail, and particularly whether it includes building the neutron elements now or in the future.

Proponents of neutron weapons within the administration and on Capitol Hill, who has been disappointed by Carter's April 7 decision , have been pressing an immediate go-ahead for two production lines - one building the low-yield nuclear shells and warheads, and the other building the neutron cores that could be fitted into these shells, making them neutron weapons.

Others within the administration want production to start now on the low-yield weapons with building of the neutron cores delayed at least for a year.

One April 7, Carter said the ultimate decision on producing neutron weapons would depend on the Soviet union and the "degree with which (they show) restraint in ". . . conventional and nuclear arms programs and force deployment . . ."

Without some time between production of weapons and their neutron cores, some officials believe there would be no incentive for the Soviets to make any arms-control gesture.

On the other side, officials have argued that without simultaneous production, it could be two years or more before neutron weapons would be available if the present decided he wanted to convert the low-yield warheads and shells.

They also have argued that if the Soviets make any response, other than saying they will not build neutron weapons, it would come only after they saw the United States was going ahead with production.

The Department of Energy could not decide between the two production alternatives.

Shortly after the president's April 7 statement, DOE officials sent the Pentagon plans for proceeding both ways. It was expected at the time that Defense would quickly approve one way or the other and production could move ahead using fiscal 1978 funds originally approved for neutron weapons by Congress in 1977.

At the Pentagon, however, questions were raised about the option that called for simultaneous two-track production of the low-yield nuclear weapons and the neutron cores.

Officials from the State Department, National Security Council, and Arms Control and Disarmanent Agency got into the discussions.

Some said that the two-track option would bring into play the Byrd-Baker amendment, attached to the Fiscal 1978 neutron money bill. It requires the president to inform Congress of a production decision and wait 45 days to give Congress a chance to veto it. Many officials wanted to avoid such a debate, fearing it could rekindle public opposition - particularly in Europe - to neutron weapons.

Concern was voiced by other officials that the two-track course also would make it appear that the president was retreating from his April 7 decision.

So the Pentagon, too, is straddling the issue, and has passed the buck to the president.

Meanwhile, the Senate Armed Services Committee, hearing of the internal dispute, wrote language in its report on the bill authorizing nuclear weapons production calling for production of the neutron cores along with the low-yield weapons.

The possibility that the decision on neutron core production might be classified was raised at a Senate Appropriations subcommittee Monday where DOE officials were outlining the production options.

Hatfield, a subcommittee member said later such a step would make public debate on the issue impossible. He reportedly has written the president for a clarification of the situation.

Officials said the options are expected to go the president officially next week, but they would not guess when a decision would be made.