Secretary of State Alexander Haig has told U.S. Ambassador to El Salvador Robert White that the State Department no longer needs his services there, according to informed sources, making White the first career ambassador to be removed from his post by the Reagan administration.

White's removal, which the sources said is effective immediately, follows his strong public criticism during the presidential transition period of policy proposals made by Reagan advisers for dealing with the turbulent Central American nation. It also is a strong indication that Reagan and Haig intend to act quickly to change controversial Carter administration initiatives in the region.

White, who has been in Washington since Haig called him here for consultations last week, said yesterday it was "up to the administration to announce whether I have been fired or not. As far as I'm concerned, I am still ambassador to El Salvador."

A State Department spokesman said last night he was "unable to confirm" that White had been told not to return to his post.

But one informed source said that a meeting of White and Haig last week was brief and to the point, with Haig saying the Reagan administration wanted to "move very quickly" on the matter. When White suggested that face could be saved all around by announcing his new appointment to another position immediately, Haig said he was not interested and that the question of White's future in the Foreign Service would be dealt with later.

Under a 1946 law governing Foreign Service positions, career officers of ambassadorial rank technically cannot be "fired." Once they are removed from a post, an administration has 90 days to assign them to a new embassy or a job with equivalent rank. Long-time department officials said yesterday they could remember only a "handful" of cases in which no new post had been offered or a "make-work" job for an out-of-favor ambassador had not been accepted.

But considering the months-long enmity between the Reagan foreign policy team and White, who had accused the Reagan team of making the Salvadoran crisis worse with hard-line rhetoric, his case may be added to that elite handful, the sources said.

One State Department official said Haig's action was "almost unheard of" in the protoco-conscious world of the diplomatic bureaucracy, where career officers whom a new administration wants transferred -- whether for policy or political reasons -- generally are given a grace period at least until they are given a new job or until their successors have been named.

Sources said the move was also surprising because the top post in the Latin America Bureau remains vacant since former assistant secretary for Latin American affairs William Bowdler was told to leave by the Reagan team early in January. He had announced his retirement.

Although four ambassadors in the hemisphere have been told they must leave within the next several weeks, all are Carter administration political appointees, and officials said they had been assured that no career officers would be charged until Haig named someone to head the bureau.

Despite continuing crises in Poland and the Persian Gulf, Central America has been viewed as a crucial test area for the new administration's foreign policy. Under the Carter administration, the region was a focus for human rights concerns and a professed respect for "ideological pluralism" that would enable the United States to deal with leftist revolutionary governments such as the one in Sandinistaled Nicaragua.

Immediately following Reagan's election, conservative spokesmen and advisers to the incoming administration publicly expressed their disagreement with Carter policy and their concerns that Nicargua had become a staging area for Cuban and Soviet expansionism in the hemisphere.

Much of that concern centered on El Salvador, where Carter had backed an ostensibly centrist civilian-military coalition junta struggling for survival against attacks by extreme rightists both within and outside the government and a guerrilla left that has now declared the country in civil war. g

White, a former ambassador to Paraguay and a career officer identified with the Carter administration's human rights policies, was appointed to El Salvador last spring in hopes he could manage the difficult task of pushing the military-dominated government toward reform while holding the line against the left.

His eventual confirmation by the Senate was delayed for weeks by conservative Sen. Jesse Helms (R-N.C.) and his prinicipal foreign policy aide, John Carbaugh, who charged at that time that White was soft on the left.

In early December, a leaked report written by foreign policy advisers on the new administration's transition team charged that both White and Ambassador to Nicaragua Lawrence Pezzullo -- both of whom had been given board leeway by Carter in helping to formulate U.S. policy in the region -- were "social reformers" and should be replaced.

In addition, a number of Reagan team members and associates said that the new administration's policies in those countries should concentrate more on stopping the left before pushing for reforms. They recommended that U.S. aid should be ended to Nicaragua's leftist government and increased to El Salvador's tenuous right-of-center coalition.

Both White and Pezzullo questioned those proposals and subsequently charged the Reagan team with "undermining their authority." Some Carter spokesmen went as far as to charge the Reagan advisers with endangering White's life in assassination-ridden El Salvador by labeling him a lame duck who would be thrown out of his job along with his policies.

The sources said no one has been chosen yet to replace White and that a temporary "acting ambassador" would be named in the interim.