Still-secret budget plans call for the Carter administration to back away from its own program for building up the Navy fleet, an about-face bound to draw fire from Congress and probably from GOP standard-bearer Ronald Reagan.

Defense Secretary Harold Brown told Congress as recently as January that the Navy ought to build 19 ships in fiscal 1982. But that number drops as low as 12 ships in new budget guidance he has sent to the Navy.

Navy Secretary Edward Hidalgo, in an internal memo, warned Brown that the new shipbuilding schedule is "most disturbing and certain to arouse determined congressional reaction."

But Brown, in a secret document called a Program Decision Memorandum, told the Navy: "I urge you to bear in mind the fiscal realities we face."

The Carter administration is faced with the dilemma of delivering on its promises to build up the nation's defense -- which means spending more money -- without at the time increasing the budget deficit and inflation.

On top of that, the Pentagon has been stung by congressional criticism of its penchant for spending too much money on new weapons and not enough on keeping those it already has in fighting condition. At any one time, for example, only about half of the nation's warplanes are ready to go to war.

In hopes of improving combat readiness, Brown and his civilian deputies ordered the Navy and other services to put more money into readiness accounts -- such as funds for spare parts -- in drawing up five-year budget plans going to Congress next year for fiscal 1982 through 1986.

Hidalgo, while following what he termed Brown's "mandatory" guidance to spend more money on readiness at the expense of building additional ships, said the resulting program may not be big enough to execute national strategy.

"While making considerable improvements in readiness," Hidalgo wrote Brown in discussing the Pentagon's budget guidance for fiscal 1982 through 1986, "the Program Objective Memorandum does not allow, except at the enhanced level, many of the modernization and equipment replacement programs that are important to our national security and essential to the execution of our strategy.

"This is a matter of great concern," Hidalgo continued. "In particular, we will build only 84 ships in the 1981-85 years rather than the 97 proposed to the Congress for those years" if the Pentagon's mid-level shipbuilding program is adopted.

The Pentagon, in putting together the president's defense budget, sets three levels for such programs as shipbuilding, called "minimum, basic and enhanced." The basic, or middle, level is the one usually adopted.

Brown's office has tentatively approved 12 new ships for fiscal 1982 as the minimum, 16 as the basic level and 18 as the enhanced budget.

Hidalgo complained in his memo that "funds above the basic level are imperative if we are to achieve the force posture needed to support our strategy."

Reagan has made Carter's defense policy a central issue in his campaign and is expected to cite the size of the Navy fleet in his attacks on the president. The Republican Party platform asserts that Carter "cut President Ford's proposed shipbuilding plan in half," and claims the Soviets will gain "worldwide naval supremacy in the 1980s unless current trends are reversed immediately."

Some Navy officials blame their service's plight on the administration's political gamesmanship, declaring Pentagon civilians are counting on Congress to add ships to the new budget.

"Adm. (Thomas B.) Hayward isn't sitting up there saying he can get by on 12 ships," said an official, referring to the chief of naval operations.

Here are the 12 ships that would be built in fiscal 1982 under the "minimum level" plan approved by Brown but subject to revision between now and January:

One Trident missile firing submarine. (Same as Brown's January plan.)

One SSN688 nuclear attack submarine. (Same as the January plan. Congress authorized two for fiscal 1981 and is likely to insist on the same number for fiscal 1982.)

One GC47 guided missile cruiser armed with Aegis antiaircraft weapons. (Brown's January plan called for three of these ships which the Navy considers vital for meeting the Soviet threat.)

One FFG7 light destroyer, called a guided missile frigate, a ship built in Maine and championed by Secretary of State Edmund S. Muskie when he was a senator representing that state. (Brown's January plan called for four FFG7s. Congress has authorized six for fiscal 1981.)

One MCM mine countermeasure ship. (Same as the January plan.)

Two ARS and salvage ships. (Same as the January plan.)

Four TAGOS antisubmarine warfare ships that trail long cables of underwater microphones. (Same as the January plan.)

One TAKX cargo ship for the Rapid Deployment Force being formed to respond to conflicts in distant spots like the Persian Gulf. (The January plan called for three such ships.)

Under the more generous "basic" shipbuilding plan, Brown approved one more CG47 guided missile cruiser, two additional FFG7 light destroyers and one TAO oiler to push the fiscal 1982 new-ship total up to 16.

The "enhanced" shipbuilding program adds one CG47 to the basic budget and another FFG7, for a total of 18 ships.

Navy leaders are appealing some of Brown's decisions on their budget, including shipbuilding. Hidalgo told Brown the medium level, or basic, budget for buying ships and aircraft is $3 billion short of what is needed to carry out the five-year plan outlined to Congress in January.