The Defense Department is rushing to finish development in the next 12 months of a new, highly toxic chemical spray bomb called "Bigeye" and an eight-inch artillery shell of delivering a long-lasting, deadly nerve agent, according to once-secret congressional testimony released by the House Armed Services Committee.

The Army has already completed development of a 155-millimeter shell that delivers another nerve gas, the hearing disclosed.

Yesterday, however, Defense Secretary Harold Brown told a closed-door meeting of the Senate Armed Services Committee that it was "premature" for Congress to provide money in fiscal 1981 to start building a factory to produce these weapons.

Brown's statement reflected the White House position rather than that of the Pentagon.

Last year, Brown approved inclusion of $19.5 million in the Army's fiscal 1981 to begin construction of a chemical weapons production facility at Pin Bluff, Ark.

When the president's National Security Council learned about the funding of the first nerve gas production plant in 11 years, it asked for an interagency study including the State Department and the Arms Control and Disarmament Agency.

In 1975, the United States had joined the Soviet Union as a signer of the Geneva Protocol which in effect prohibits a nation from first use in war of deadly chemical and bacteriological weapons.

The protocol does not, however, bar development, production or stockpiling of such weapons.

In August 1976, the two superpowers began negotiation for a comprehensive treaty to ban chemical weapons. After President Carter took office in January 1977, he encouraged continuation of the talks and at the same time directed that there be no modernization of the chemical warfare stockpile.

In fiscal 1975 and fiscal 1976 budgets sent to Congress, the Ford administration sought long lead-time construction funds to build a facility to produce the new chemical munitions.

Unlike bombs and shells in today's stockpiles, which contain the deadly chemical agents already mixed, the new generation of weapons would be binary, a system that keeps the chemicals separate and harmless until they are mixed at the time they are fired.

Congress in 1975 and 1976 refused to approve the construction funds, pending the outcome of chemical warfare negotiations with the Soviets.

Over the next four years, the Carter administration did not request any funds for plant construction, though there was ongoing support for development of chemical weapons.

Last year, however, faced with reports that the Soviet Union was stepping up its production of chemical warfare arms, the Pentagon decided to go ahead with initial funding of a chemical weapons plant.

The resulting NSC study, ordered by national security adviser Zbigniew Brzezinski, determined that neither State nor ACDA would support a start on the production facility, on the ground that it would hurt negotiations with the Soviets. The Pentagon then withdrew the money from its fiscal 1981 budget request.

Last spring, however, the House Armed Services Committee put $3.2 million in the fiscal 1981 defense authorization and that amount was approved in June by the entire House.

Brown's testimony yesterday is not expected to deter the Senate committee from approving at last the same amount that passed the House and maybe more.

Conservative members of Congress are pushing for production of the new generation of chemical weapons. Allegations of Soviet use of such devices in Afghanistan are one of their major arguments.