The Reagan administration, finding no signs of increased support in Congress for renewed aid to the Nicaraguan contras, may delay until January its request for $270 million in new military assistance for the rebels, U.S. officials said yesterday.

The officials, stressing that no decision has been made, acknowledged that White House spokesman Marlin Fitzwater had signaled the possibility of a change in the administration's plan when he said that "there might be some change in the timing of the final request."

Fitzwater's statement that the administration is "flexible" about when it will seek a congressional vote on contra aid contradicted past assertions by Secretary of State George P. Shultz, who has said that President Reagan intends to send the $270 million request to Congress before Thanksgiving.

However, all five Central American presidents involved in the peace plan have said that their agreement calls on outside countries to stop aiding insurgent forces in the region at least until January, when the presidents are scheduled to assess whether the plan is being implemented successfully. The peace plan is supposed to take effect Nov. 5.

The officials said that if the administration proceeds with its aid request in the face of this solid front, it might open itself to new charges that it is trying to sabotage the plan because of its hostility to Nicaragua's Marxist government.

The officials added that the position of the Central Americans is certain to be echoed in Congress where House Speaker Jim Wright (D-Tex.), an outspoken champion of the peace plan, has threatened to block any request for new contra aid until the agreement has been given a chance to work. They also acknowledged that the administration is reluctant to challenge Wright at this time because its soundings indicate that congressional hostility to contra aid remains high in the wake of the Iran-contra hearings.

In addition, the officials continued, Congress and the administration are under pressure to reduce the federal deficit as a result of the plunge in the stock market. That has given the administration second thoughts about the timing of a contra aid request. They said that congressional leaders might cite the need to deal with financial crisis as a reason for putting off the contra aid issue indefinitely or even for voting against it as an unnecessary expense.

For all these reasons, the officials said, many administration policymakers are leaning toward making the aid request in January, after a better picture emerges on whether the Nicaraguan government is complying in good faith with the plan's requirements for democratization and reconciliation with its domestic opposition.