Fearing that the two new missiles basic to NATO's nuclear modernization program and the president's arms reduction plan will not be ready for their scheduled December, 1983, deployment date, the Pentagon has decided on the costly and technically hazardous procedure of beginning production before their development and testing programs are completed.

Top Pentagon officials say this "concurrency program" is necessary because of concern that any delay in the Pershing II and ground-launched cruise missile systems could increase Western European opposition to their deployment.

"The program does have risks in it in the context of how much concurrency has been accepted," Deputy Undersecretary of Defense James P. Wade Jr., told a closed hearing of the House Defense Appropriations subcommittee in September, the transcript of which was recently released.

Pressed, Wade characterized the risks of meeting deployment schedules as "moderate."

"If we say that we think it is going to slip," he said. "I think that will be sure to take place . . . ."

Thereafter, Wade went on, "other elements of the program will then go diving under the tent."

American determination to push the systems in this faster-than-normal procedure, Wade said, was to meet "a commitment to our NATO allies" made back in December, 1979.

At the time the diplomatic commitment was made, however, neither system was expected to be ready by the December, 1983, date.

The Pershing II, for example, was in a development program that had December, 1984, as its target date. And the ground-launched cruise missile program, which had already run into technical problems, was being deemphasized in favor of a submarine-launched cruise missile.

As explained at the congressional hearing, the concurrency programs for both missiles will set up the following problems:

* The Pershing II will have undergone only two of its planned 28 flight tests before the Pentagon must make its production decision in June, 1982. Those two tests will be the first of the missile at its new, extended 1,000-mile range. Up to now the Pershing had a 400-mile range.

The first flight test of the Pershing II against a land target is not scheduled until July, 1982. The testing of an operational, mobile Pershing II will not take place until mid-1983.

Pentagon officials believe the new guidance system on the Pershing II was adequately tested during 1977-78 shots carried out with Pershing I missiles.

* The ground-launched cruise missile, which has already slipped two years in its testing program, has a scheduled production decision date of May, 1983, by which time more than one-third of the procurement funds will have been spent.

The delay, in part, came from troubles the Air Force has had with the computer system for the missile.

"The problems we have run into have been in the development of the software for the computer," Maj. Gen. R. D. Russ, Air Force deputy chief of staff for research and development, told the subcommittee. "I think maybe we were a little ambitious when we started," he said, adding that he believed if the 1983 date were to be reached, "we need to proceed at the pace that we are going."

There has been only one test to date of a prototype transporter for the cruise missile that led to a redesign of the equipment, the congressmen were told. "That is the way we planned it," Russ told them. "We do not plan on building a full-scale development engineering model of the transporter until this year."

Testing of the missile itself is being done primarily through continued testing of the submarine-launched cruise missile. It has been tested 58 times, the congressmen were told, and "is about 99 percent identical to the land-attack vehicle."

From an originally estimated cost of $1.5 billion, the cruise missile program, according to Rep. Joseph M. McDade (R-Pa.), would end up at about $3.7 billion when it is completed.

Development and production costs of the Pershing II program has grown at a much slower rate, from $1.5 billion in fiscal 1979, to $1.7 billion in fiscal 1982, with all growth attributed by the Army to inflation.