The Navy has so few combat planes that it could not replace those it expects to lose in the first days of a war.

"This is a serious shortfall," Adm. James D. Watkins, chief of naval operations, said in a memo written to Navy Secretary John F. Lehman Jr. to influence future Navy budget requests.

The House Appropriations subcommittee on defense, which obtained the memo during recent hearings, is citing it as proof that conventional forces are receiving short shrift under President Reagan's proposed defense buildup.

Congress is expected to continue debate on Reagan's defense budget when it takes up the fiscal 1984 Pentagon procurement bill Monday.

Why Navy combat planes are so scarce in Reagan's record-high peacetime budgets is a question hanging over his rearmament program.

Congressional foes of the MX missile intend to focus on that question in contending that the cost of giant weapons such as the MX leaves too little money for aircraft, tanks and manpower for conventional warfare.

"When the members understand how the big-ticket items like the MX are taking money from other programs like Navy aircraft," said MX opponent Rep. Joseph P. Addabbo (D-N.Y.), chairman of the defense panel, "they will be less likely to vote for it next time."

Addabbo plans to use Watkins' memo on aircraft shortages in trying to persuade House members to deny money needed to put the MX into production.

The Watkins memo, sanitized for security reasons for public debate, states: "Today we have virtually no combat attrition aircraft, while our own exercises and projections show attrition from the first day" of a war. "It does little words censored if we run out of aircraft . . . . This is a serious shortfall."

Pentagon leaders admit to being caught in the same vicious cycle that has plagued previous administrations on buying Navy aircraft. As aircraft orders are reduced to free money for super-weapons and other demands, savings through mass production are lost, raising the price of each plane and reducing the number of aircraft that can be purchased for a given amount.

Reagan's fiscal 1984 defense budget, for example, calls for buying only six A6 light bombers at $40 million each, including part of the development cost and spare parts.

The recommended F14 fighter buy is 24 planes at $50 million each, while each new F18 fighter-bomber, billed originally as the low-cost alternative to the F14, is pegged at $33 million, thanks in part to a larger production order of 84 planes.

The Navy goal is to buy 330 aircraft a year to offset losses from accidents and old age and to keep the planes' average age at 14 years.

"At no time in the past 10 years has the Navy purchased over 300 aircraft in any one fiscal year," Watkins recently told the House defense panel.

Even in peacetime, the Navy has had a tough time surmounting aircraft losses. In fiscal 1978, the Navy lost 467 planes and received funds to buy 181. In fiscal 1982, under a generous Reagan budget and reduced accident rate, the Navy gained a little ground, losing 241 planes but getting appropriations for 287 replacements.

Deputy Defense Secretary Paul Thayer, who came from the aerospace industry, and Lehman are resorting to extraordinary measures to try to solve the problem.

One idea is to save money by shutting down some aircraft production lines. Another is to threaten to do so to pressure contractors to lower prices.

The guinea pig for this proposed experiment is Grumman Aerospace Corp. of Bethpage, Long Island, manufacturer of the F14 and A6. In its program objectives memo for the fiscal 1985 defense budget now in preparation, the Navy recommends shutting down Grumman's F14 line from fiscal 1986 through 1988 and the A6 line for fiscal 1987 and 1988. The lines would be restarted after the suspensions.

Grumman and Addabbo, stout protector of jobs on his home ground of Long Island and a Grumman champion in Congress, are protesting that plan. Lehman's advice to Grumman is to make him an offer he cannot refuse, such as a lower price in exchange for keeping the line open.

The specter of a Navy without enough planes to fight beyond initial battles is forcing Congress to address not only how much is enough for defense but also how it should be divided. The $6.7 billion budgeted for Navy combat aircraft in fiscal 1984 is $400 million less than the amount approved last year for two Nimitz-class aircraft carriers.

"Because of all the money going for such big-ticket items," Addabbo complained in an interview, "we're back in the hills and valleys of procurement."