The Reagan administration misleads the public by asserting that a presidential task force believes draft registration would save as much as eight weeks in mobilizing for war, two representatives of anti-registration groups charged yesterday.

Barry Lynn, president of Draft Action, an anti-draft pressure group, and David Landau, an American Civil Liberties Union lawyer, made that charge as they released the previously confidential report on draft registration prepared for President Reagan by his Military Manpower Task Force.

Dated Dec. 15, 1981, and declared authentic last night by the White House press office, that report makes no recommendation, but cites four options, ranging from keeping compulsory registration in force to waiting until after declaration of a national emergency requiring mobilization.

Reagan had opposed compulsory registration during his election campaign, but reversed himself last Jan. 7, saying of the task force chaired by Defense Secretary Caspar W. Weinberger:

"On the basis of their findings, I have decided to continue registration . . . In the event of a future threat to national safety, registration could save the United States as much as six weeks in mobilizing emergency manpower."

In public statements since then, administration executives have often stretched that figure to eight weeks. Lynn charged that this amounted to "lying to the public about the need for this system."

Joan Lamb, speaking for the Selective Service System, acknowledged that the six-to-eight-week estimate has been voiced and said it stems from a growing appreciation of the complexities of registering men after mobilization, not from an intent to overstate the case for compulsory registration.

Landau, noting that the report Reagan cited is a list of pros and cons of various options rather than a firm recommendation by the task force, said, "We now find that the supposedly compelling case for this system doesn't exist."

Lynn and Landau said Congress should reconsider the current law which requires young men to fill out registration cards at local post offices within 30 days of their 18th birthdays.

Lamb said Reagan chose the option of compulsory registration because it promised to be the surest way to respond to a national emergency.

The report said six weeks would be lost in getting young men to induction centers if compulsory registration were abandoned.

Under compulsory registration, the stream of draftees could be started on their way in two weeks.

Eight weeks would be needed if registration were delayed until after mobilization.

If registration forms were stored during peacetime in 250 postal centers around the country and then distributed "immediately upon mobilization" to 35,000 post offices, it would take four weeks to deliver the first draftees for processing -- or two weeks longer than the compulsory system now in force, the report said.

The task force added that the president could discontinue compulsory registration and resume it once a crisis seemed imminent.

However, the panel warned that this might "escalate the crisis" and that "congressional and public support for resuming registration during an international crisis may be uncertain."

In discussing the symbolism of requiring young Americans to register for the draft, the task force said:

"The Joint Chiefs of Staff believe peacetime registration is an important aspect of U.S. deterrent strategy . . . All North Atlantic Treaty Organization allies, except Canada and the United Kingdom, have conscription and would likely view our stopping registration as inconsistent with our policy of urging them to make greater defense efforts . . .

"Since President Carter initiated registration as a result of the Soviet invasion of Afghanistan, the Soviets might perceive the elimination of registration as a partial acquiescence to the occupation of Afghanistan.