The Navy has been devoting such a large chunk of its budget to buying the highly sophisticated and expensive F-14 Tomcat fighter that it has not procured enough aircraft "to prevent excessive and eventual decline in force levels," according to a secret memorandum prepared by the staff of Defense Secretary Harold Brown.

The memo also indicates that Brown's staff believes the Marine Corps is trying to move ahead too rapidly with purchases of airplanes, known as VSTOLs, that can take off and land vertically.

The memo is one of a number coming to Brown as part of a program he has undertaken to force the services to get more weapons for their money and to operate with fewer people. The program, reminiscent of one undertaken by Defense Secretary Robert S. McNamara in the early 1960s, is being imposed on the services by Brown as he prepares their budgets for the next five years.

According to one memo obtained by The Washington Post, so much, Navy money has been going to buy the F-14 - at $19 million a plane - that all during the 1970s the fleet has been getting only about one-third the number of fighter and attack planes it has needed.

"Since 1976," the secret memo states, "the Department of the Navy has not procured enough fighter/attact aircraft to prevent excessive and eventual decline in force levels . . ."

Instead of the "minimum necessary" purchase of 180 planes a year to fill fleet needs, the memo complains, the Navy has been buying only about one-third of that, including 56 in the current fiscal year and a "not much improved" request for 57 in the fiscal 1979 budget now in preparation.

The Navy has been buying "roughly 50" F-14s a year instead of more copies of cheaper planes, the memo says. The resulting shortages are worser than they look, the memo states, because the number of carrier air wings was reduced from 15 to 12 between fiscal 1973 and 1978.

Besides putting a disproportionate amount of money into the F-14 purchases, the memo complains, Navy leaders have been scattering their remaining aircraft dollars over too many dirrerent makes of planes.

The Navy flies "12 models of six different aircraft types," the memo states. "The consequences of this policy are inefficient production rates and high unit costs. Increased commonality abd a reduction in the different types of aircraft" in the Navy's "fighter attack force must be accomplished" if squadrons are to kept up to strength, says the memo.

Defense Secretary McNamara, for whom Brown worked as Pentagon research director and then Air Force secretary in the 1960s, used to order the same kinds of studies that are now going into Brown's in basket. The lack of "commonality" decried in the memo recently done for Brown was part of the impetus behind McNamara's insisting that the Navy and Air Force build a common fighter called the TFX and later the F-111.

However, the predicted savings from building one fighter for both services were not realized as the Navy broke off from the program and started to build the F-14 drew fire from Deputy Defense Secretary William Clements during the Ford administration. Clements told Navy leaders they either had to build a plane cheaper than the F-14 or do without any new carrier planes at all, according to Pentagon sources.

The Clements warning was part of the pressure to build the F-18 fighter for the fleet. Now the F-18, being built by McDonnell-Douglas and Northrop, is experiencing such severe technical and money problems that Pentagon analysts are recommending everything from delay to cancellation.

To address the problems of buying the planes the fleet needs, Brown's advisers wrote wrote that he had three basic choices: buy more planes in a hurry to fill current gaps; or "immediately and permanently reduce" the number of planes assigned to the fleet so as not to break through budget guidelines.

To the distress of many Navy leaders and the affected airplane companies, sources said that Brown has tentatively decided to cut a lot of F-14s from the Navy's shopping list; cancel the Navy's planned purchases of the A-7 attack plane, and delay development of the F-18 for about a year.

Some of the gaps in the Navy's fighter and attack aircraft force would be filled by buying more A-6 attack planes than the Navy had intended in its budget plan covering fiscal 1979 through 1983.

In other actions that some Navy leaders portrayed as the biggest blow yet to their plans to sitch from the giant Nimitz-class to smaller "midi" aircraft carriers, Brown is recommending a lower entry into the age of vertical and short take off and landing aircraft. His misgivings about VSTOL, a technology that proved disappointing in Brown's earlier Pentagon tour, have prompted him to slash the Navy's proposed five-year VSTOL budget from $1.2 billion to $550 milliom, Pentagon sources said.

At the same time, Brown is recommending a slowdown in the Marine Corp's march into the VSTOL era by denying money for the British-designed Harrier, both the version now flying and an improved version scheduled to be delivered soon.

If Brown's slash of VSTOL development money sticks through the budget process underway, one congressional action can be predicted with certainty. House Armed Services Committee critics of abandoning the $2 billion Nimitz carrier for the "midi" before a new generation VSTOL plane is ready would argue that any Pentagon-ordered delay in VSTOL development of the Nimitz.

Navy Secretary W. Graham Claytor, sources said, is among those distressed by Brown's proposed slow-down in VSTOL planes for the "midi."

Also, Claytor is said to favor scrapping the F-18 to enable the Navy to buy more F-14's and other sophisticated planes.

In reviewing the Marine's plans for buying more Harriers for troop support, Brown is proposing that the corps buy more of the proven McDonnell-Douglas A-4M attack planes and cut back on purchases of the Harrier. There is a chance under this revised plan that the Marines would not get the advanced Harrier, the AV-8B, at all, but this has not been decided, sources said.

Navy leaders who want to keep buying large quantities of the F-14 argue that the Soviet threat to the fleet dictates that this fighter be purchased even though it means going without as many new aircraft as the fleet needs. But Brown's recommended cancellation of the Air Force B-1 bomber and other budget actions indicate that he believes a weapon can price itself right out ofthe U.S. arsenal.