The Pentagon plans to spend more than $500 million a year for the next several years to help the defense industry prepare to swing quickly into wartime production, officials said yesterday.

Pentagon executives said the nation's defense contractors are in no condition to do that today, undermining the ability of the United States to wage a long conventional war. Fred C. Ikle, undersecretary of defense for policy, in an interview yesterday, termed the shortcoming a "deep concern."

To correct the deficiency, Ikle said, the Pentagon is considering a full range of measures, including financial incentives to build extra defense plant capacity and the recruiting of extra workers for emergency, around-the-clock production.

Another proposal under discussion is the building of prototypes for weapons and related equipment that could, in an emergency, be produced quickly. What the Pentagon has in mind, officials said, is something like the simple but efficient Liberty cargo ships that shuttled material from the United States to Europe during World WAR II.

The idea of a big push to beef up the defense industry in peacetime is controversial and has not become official policy. Critics contend the approach could mean wasting millions of dollars.

Ikle is a strong proponent of preparing for extended conventional wars rather than accepting the theory thatr any next conflict will be brief. This week he installed Sol Love, 61, a former aerospace executive at the Vought Corp. to coordinate Pentagon efforts to gear up the defense industry.

Asked how much money the Pentagon plans to earmark for gearing up the U.S. defense industry for an emergency, Ikle said that "one-half of 1 percent of the Pentagon's budget would be a reasonable premium to pay for that insurance."

Pentagon budgets are scheduled to increase by at least 7 percent a year, after allowing for inflation, over this year's total of $222 billion. This means that the cash incentives for the defense industries to prepare for a surge of orders would come to more than $500 million a year for the next several years.

Ikle said that standby industrial capacity to support extended conventional warfare would help deter conflicts in likely trouble spots outside of the North Atlantic Treaty Organization, with the Persian Gulf the leading case in point.

If war did break out, he maintained, the ability to bring U.S. industrial might to bear quickly would help win it without resorting to nuclear weapons.

Defense Secretary Caspar W. Weinberger has given his blessing to the efforts of Ikle and others to beef up the military's industrial underpinnings. In his recent guidance to the military, Weinberger told it the defense industry should have the ability to absorb as much as half the gross national product during an emergency.

Weinberger said that he had a wartime emergency in mind when he issued that guidance.

There is also a school of thought, however, that argues the administration should prepare for massive increases in the defense budget in response to a period of tension when no shots are being fired.

The "short warning conflict scenarios" of the past have "ignored the essential need to ensure a broad and flexible defense industrial base that could absorb rapid, massive increases in defense budgets . . . in response to a dramatic, shift in the world situation short of conflict," said one internal Pentagon paper.

"Those goals clearly require a broadened approach to industrial mobilization planning -- one that is decoupled from the present (or possibly any) conflict scenarios."

While Ikle and his deputies are addressing the policy problems of improving the industrial base, Richard D. DeLauer, undersecretary of defense for research and engineering, is concentrating on specific steps that would help the contractors prepare for filling emergency orders.

He recently lamented the fact that there are 80 different laws administered by 20 different federal agencies that cover raw materials used by the defense industry.

Critics of the Pentagon's push to improve the industrial base warn that billions would have to be spent to stockpile critical metals like cobalt and chromium. They also question whether there would be enough skilled manpower to quickly increase the production of such sophisticated conventional weaponry as fighter aircraft. t