The CIA is planning to support a rapidly growing "secret army" of 12,000 to 15,000 anti-government rebels in Nicaragua, roughly double the number backed by the United States two months ago, official sources said yesterday.

The sharp increase in planned U.S. support comes as the House of Representatives moves toward a legislative showdown, probably next week, on continuing undercover activity in Nicaragua.

The House intelligence and Foreign Affairs committees have approved a bill sponsored by their chairmen, Edward P. Boland (D-Mass.) and Clement J. Zablocki (D-Wis.), respectively. It would terminate secret U.S. aid to the insurgents and authorize an open $80 million program to stop leftist gun-running in Central America.

A secret House session to discuss the proposed cutoff, opposed by the Reagan administration and most House Republicans, is scheduled Tuesday with an open vote to follow. House Democrats have scheduled a closed caucus today to discuss this and other politically sensitive issues regarding Central America.

The mushrooming growth of the U.S.-supported insurgency against Nicaragua's leftist Sandinista government has generated much of the controversy and, in some quarters, consternation on Capitol Hill.

In early May, the CIA told congressional oversight committees that the U.S.-supported rebel forces had swelled to 7,000 men. By early June, the official estimate had climbed to 8,000, and last week the State Department officially estimated the force at 8,000 to 10,000.

In recent days, according to the sources, the CIA has drawn up a plan to support a force of 12,000 to 15,000 with money and materiel and is seeking presidential authorization for such expanded activity. A CIA spokesman declined to comment on the matter yesterday.

A U.S.-supported force of 12,000 anti-leftist guerrillas in Nicaragua would be at least twice the reported size of the leftist guerrilla force opposing the government of El Salvador. The State Department estimates that 4,000 to 6,000 leftist rebels, aided by Cuba and Nicaragua, are operating in El Salvador against the U.S.-supported government.

During most of the early buildup, the principal U.S. justification for supporting anti-Nicaraguan insurgency was that it could reduce or end Nicaraguan military assistance to Salvadoran guerrillas. However, this justification has been receding as the U.S.-supported "secret army" has grown larger than the Salvadoran insurgency.

According to one account, a new presidential "finding," or secret intelligence authorization, being prepared by the CIA no longer lists interdiction of arms as one purpose of the undercover war in Nicaragua.

Instead, this account said, the stated purpose is to force changes in Nicaraguan government policies, including those of aiding leftist guerrilla forces elsewhere in Central America.

Last December Congress passed an unusual law, known as the Boland amendment, banning U.S. aid to paramilitary forces "for the purpose of overthrowing the government of Nicaragua or provoking a military exchange between Nicaragua and Honduras."

The administration has denied its purpose is to overthrow the Nicaraguan government, although leaders of rebel forces supported by the United States there have said they are trying to topple the government. That has prompted some members of Congress to charge the Boland amendment is being violated.

In initiating the secret effort in December, 1981, the CIA told congressional committees it was building a highly trained commando force of 500 Latins to attack the Cuban support structure in Nicaragua. Some lawmakers immediately expressed concern, and it was revealed later that Boland addressed a confidential letter to CIA Director William J. Casey about Hill disquiet.

According to official estimates, most of the U.S.-backed Nicaraguan guerrillas are in a group near the Honduran border. They are composed of separate groups of Miskito Indians and exile-led insurgents known as the Nicaraguan Democratic Force. About 1,200 guerrillas are reported near the Costa Rican border and commanded by Nicaraguan exile leader Eden Pastora. Despite reports to the contrary, Pastora is fighting against Nicaraguan forces and receiving U.S. support, the sources said.