President Carter's plan for registering young men and women for the draft was termed "redundant and unnecessary" by the Selective Service before he adopted it, according to an internal report obtained yesterday.

In his 29-page study sent to the White House, Selective Service Director Bernard D. Rostker said his agency could carry out the war plans without resorting to registering people in advance of a national emergency.

Carter ignored this advice. His call for registering this year all men and women born in 1960 and 1961, has touched off a national controversy.

Critics of Carter's registration decision are demanding that the administration release the report, which was obtained by The Washington Post. The American Civil Liberties Union and Rep. Patricia Schroeder (D-Colo.) have gone so far as to file papers under the Freedom of Information Act to get the document.

The study, dated Jan. 16 and stamped "draft" copy on its cover, makes these points, which are sure to fuel the national debate over registration:

Registering people after a national emergency is called, rather than ahead of time as Carter has decided upon, "is preferable" and "should substantially exceed defense requirements, employs the fewest number of full-time personnel and costs the least."

"While costs and staffing should not be the determining factor, the reduced delivery time provided by other options is redundant and unnecessary.

"The post-mobilization option" [registering people after a national emergency is called], subject to field testing later this year and the international situation at any time, is recommended as the basis for an effective standby Selective Service."

Requiring people to register at their local post offices before an emergency occurs -- the Carter plan -- would save only seven days over Selective Service's recommendation.

Under the Carter plan, the first draftees would be inducted within 10 days after an emergency was declared. Selective Service's wait-until-afterward option would take 17 days. War plans call for getting the first people inducted within 30 days of mobilization.

The same seven-day difference between the Carter and Selective Service plans would apply to the induction of the first 100,000 people -- 26 days versus 33 days after mobilization. The Pentagon wants the first 100,000 within 60 days.

Similarly, Carter's plan would save only seven days -- 117 versus 124 -- in getting the first 650,000 people to training camps. Pentagon war plans call for getting this number into uniform within 180 days, leaving a comfortable margin, as Selective Service sees it.

"We believe that we now have a capability to respond in an emergency" without resorting to advance registration. The word "now" is underlined in the report.

Selective service said it had improved its ability to act by streamlining procedures and working out arrangements with the U.S. Postal Service to rush into registration if an emergency occurs.

"The post-mobilization option is by far the most cost effective, and least intrusive, and is the option chosen by Selective Service."

John White, White House manpower chief, confirmed last night that Selective Service's Jan. 16 draft report came to him as he was preparing for Carter an interagency report on registration. But the text of the report was not sent in that form to the president, White said.

"The problem" with the Selective Service's recommendation, White said, was that it assumed that "the system is up and running and all working well."

"The world has changed," he continued. White said that by calling for registration, the president "is indicating to the world our resolve."

Also, White said, "Carter wanted to be sure the [Selective Service] system would respond. The key element is registration."

Carter called for draft registration in his State of the Union address on Jan. 23, declaring that "we must be prepared for [the] possibility" of returning to the draft.

In subsequent elaborations, Carter said he intends to register women as well as men. It will take congressional legislation to register women, but Carter can order the registration of men by issuing an executive order.

Congress seems certain to reject legislation to draft women, leaving it up to Carter to order the registration of men in an election year. Selective Service has not set a date for beginning registration, saying only that it would start in the spring.

The next formal congressional examination of the issue is scheduled for Tuesday, when a House Appropriations subcommittee will conduct a hearing on the president's request for money to gear up Selective Service for registration.

The administration is requesting an extra $21.9 million for fiscal 1980 and $35.5 million for fiscal 1981.