Despite the Army's increasing reliance on reserves in its war plans, one-third of such troops needed to support a European war would be unprepared or only "marginally" ready for combat during the crucial first month of action, according to a Congressional Budget Office report.

The report estimates that 37 percent of combat support units -- artillery, engineering and armored troops -- and 31 percent of essential medical, maintenance and transportation troops would not be fully or substantially prepared.

Throughout the reserves, despite a marked improvement in quality of recruits since 1981, "most reserve units are rated 'marginally ready' or 'not ready' for combat," the report says.

Significant shortfalls in equipment, personnel and training are especially worrisome, the CBO said, because the Army is continuing to place more responsibility on reserve units. The Army counts on reserves to provide 42 percent of its force during the first month of a European war, the CBO said.

"These results do not necessarily show that the Army could not meet its own deployment plans, which are not publicly available," the report concludes. "But the results do suggest that, under reasonable assumptions consistent with unclassified information, the Army would have difficulty in meeting requirements, especially for early-deploying support units."

A Defense Department spokesman said top officials have not seen the report and would not comment on its conclusions. But the spokesman cautioned that readiness figures cited by CBO are not entirely reliable.

The CBO, a nonpartisan analytical arm of Congress, conducted its study at the request of the House Armed Services subcommittee on military personnel and compensation. While making no recommendations, it suggests that the Army could beef up existing reserve units -- especially those that would deploy first -- instead of adding new units as is planned.

The shortfall in early-deploying units is especially important because North Atlantic Treaty Organization strategy depends on blunting a hypothetical Soviet attack in the early days of a war. Without that capability, NATO commanders have said, they would be forced to turn quickly to nuclear weapons.

Partly in response to budget pressures, the Army plans to maintain its active-duty strength at 781,000 during the next five years while increasing its part-time Army Reserve and Army National Guard strength from 724,000 to 840,000. To help finance that increase, the Army would raise the reserves' equipment and manpower budget from $8 billion to $11.2 billion.

Even that increase, if approved by Congress, could leave the reserves 25 percent short of "critical equipment" and deficient in spare parts and ammunition, the CBO said.

More importantly, many reserve units would have no experience with the most modern equipment, such as M1 tanks and Blackhawk helicopters, which they would be expected to repair for active-duty troops.

The report does not say what proportion of units the Army considers "not ready" because that number is classified. But it says the proportion of units rated at least marginally ready declined during the last two years.

Those statistics reflect in part a peculiarity of Army recordkeeping. Once a unit is slated to receive new M1 tanks, for example, it is considered unready until all of those tanks arrive.

Thus, the readiness rating of a well-trained unit with older M60 tanks might plummet from one day to the next, although its actual combat fitness would not have changed.

But congressional analysts said problems in the reserves go well beyond statistical anomalies.

"On average, reserve units are much less ready than active units to perform their missions," the report says. "Nor has this pattern changed appreciably over the past three years, despite the infusion of additional resources into the reserves."