Navy Secretary John F. Lehman Jr. yesterday told critics of the Reagan administration's planned five-year, $100 billion fleet buildup, "sorry . . . it's too late to stop it . . . we've got the 600-ship Navy."

The Navy's civilian chief made his comments in response to a statement by longtime Pentagon consultant William W. Kaufmann, a professor at the Massachusetts Institute of Technology who helped write the annual Pentagon report to Congress for nearly every secretary of defense in the 1960s and 1970s.

Kaufmann estimated that the United States can handle the military problems it faces around the world with an annual growth rate in defense expenditures of about 6 percent instead of Reagan's projected 7 to 8 percent, at a five-year savings of about $250 billion.

Speaking at an all-day seminar on military spending at the Brookings Institution, Kaufmann said that his first priority for reducing Reagan's $1.6 trillion five-year defense plan would be to squelch the Navy's "ambitions for a 600-ship" fleet. That ambition is "not only not feasible within the existing program but it is also not necessary," he declared.

"Professor, I'm sorry to suggest to you that it's too late to stop it," Lehman countered. "We are already up to 514 ships (counting some newer vessels in the reserves) and we've got 106 more ships under contract . . . more under contract than at any time since the Korean War. So it not only is affordable, it is being done."

"We accomplished it," Lehman continued, by "front-loading," meaning going to Congress and then the defense industry quickly to get a large number of ships authorized and under contract in the early Reagan defense budgets. People forget, he added, that "we are now in the fourth Reagan budget," a reference to the budgets for this year and last, plus other revisions.

Furthermore, Lehman claimed, the last 15 ships delivered are coming in ahead of schedule and under cost because of the economies of large-scale production.

So unless there are cuts in the Navy budget beyond those being advocated by many critics, Lehman said "we are essentially there" with the 600-ship fleet in service by 1989.

Kaufmann, however, said the way he looked at the numbers, the Navy has already committed $35.1 billion and there is "still $77.4 billion to go to get to the mythical 600 ships. So the commitment is by no means made . . . there is still ample time" to cut it.

Lehman argued that much of what's left was for smaller vessels whose deliveries come after the 600-ship level is reached.

Philip A. Odeen, a former White House and Pentagon official, voiced concern that the Navy's great emphasis on procurement now would squeeze out funds for combat readiness years from now when all the construction bills come due.

"We'll have a terrible time," Odeen warned.

Lehman said the bigger Navy was not a result simply of admirals pushing for it.

"We can get along with a smaller Navy," he said, but only if an American government decides to cut down on treaty commitments with some 40 nations around the world and otherwise pulls back from areas of interest. In the meantime, he argued, it was the Navy's job to fight and win wherever it may have to and, given the expanding Soviet fleet, it was impossible to do that with any confidence with fewer than 600 vessels.