Former secretary of defense Robert S. McNamara and two fomer U.S. arms negotiators said yesterday that President Reagan is risking an accelerated nuclear arms race, in which the Soviet Union has an advantage, by abandoning the unratified SALT II treaty.

"Without SALT, the entire structure of offensive arms limitation which has been laid down over a period of 15 to 20 years by four presidents -- Presidents Johnson, Nixon, Ford and Carter -- will be destroyed," said McNamara at a news conference called to oppose the decision announced by Reagan Tuesday.

Backed up by Gerard C. Smith and Paul C. Warnke, who were the chief U.S. negotiators of the 1972 SALT I treaty and the 1979 SALT II treaty, respectively, McNamara said that "a totally unconstrained arms race" could result from Reagan's decision.

Such a race, the former officials declared, would harm U.S. national security because the Soviet Union is in a better position to build additional nuclear missiles more quickly than the United States. The former officials also said the Soviets are not likely to continue taking older weapons out of service as new ones are deployed, as required by SALT II, if the United States is no longer doing so.

State Department spokesman Charles E. Redman, in response, said, "While the Soviets could add still more weapons, it's difficult to understand what practical purpose such expansion could serve. What is more, the Soviets would be able to undertake substantial growth in their offensive forces even under SALT II. In the absence of SALT II, this would not necessarily be appreciably different from that which would occur if they were in compliance with the agreement."

Asked to explain the last remark Redman said the Soviets are expected to continue their missiles modernization program even under the SALT II treaty limits. Should the Soviets stop taking older missiles out of service, "these aging systems are not necessarily of military significance," Redman said.

At the the news conference sponsored by the Arms Control Association, Warnke said the Soviet Union could add more than than 6,000 additional ballistic missile warheads on their SS18 missiles if released from the SALT II limit of 19 warheads for each of these giant missiles.

The Soviets could gain additional strength by not retiring SS17, SS18 and SS19 missiles as they deploy new SS24 missiles, Warnke added. According to data given reporters at the news conference, at least 1,360 Soviet strategic nuclear delivery systems have been dismantled, destroyed, withdrawn or converted to other uses to comply with the SALT treaties.

"What we are facing now is Soviet action that in a relatively short period of time and with no massive expenses would add probably twice as many intercontinental ballistic missile warheads as they have at the present time," Warnke said.

Warnke charged that Reagan's decision "opens the floodgates for massive Soviet increases" and "is inconsistent with U.S. national security."

Smith charged that Tuesday's announcement represented "a sort of ideological decision reflecting pressures from people who don't like arms control."

McNamara, under questioning from reporters, said, "I don't think there is any doubt" that the Soviets have violated some provisions of the SALT II treaty, but added that none of the violations has changed the strategic nuclear balance with the United States. Reagan's response to Soviet violations, he added, is "totally inappropriate."