The Reagan administration yesterday backed off from efforts to eliminate the antitrust authority of the Federal Trade Commission, but insisted that the agency come up with similar, potentially massive cuts in its budget.

The decision was announced by Sen. Robert Packwood (R-Ore.) after a private White House meeting with President Reagan and Martin Anderson, Reagan's top domestic policy aide.

According to Packwood, Anderson said it was "never our intention" to eliminate the Bureau of Competition, the arm of the FTC that shares antitrust enforcement with the Department of Justice. "Whether it was or not, I'm not going to argue," Packwood said."At the moment, that is no longer their plan."

Packwood's description of the White House conversation comes despite a Feb. 7 memo from the Office of Management and Budget to the FTC which lists as one of the "proposed changes" the 36-month "phaseout" of the FTC's "maintaining competition mission." The memo also refers to the budget savings resulting from the "elimination of the antitrust enforcement in FTC."

An administration source said the White House "simply can't deny that that was a propsal," but reiterated that, according to the new plan, the cuts in FTC funding "would be largely at the discretion of the agency, rather than explicitly eliminating a given function."

But the administration change of position is anything but a victory for the FTC, which is now faced with working out massive across-the-board budget cuts with Congress. Although the commission volunteered in a letter to OMB Director David Stockman dated Feb. 14 to find budget cuts totaling 10 percent for each of the next two fiscal years, the cuts proposed by the OMB are far broader.

The OMB proposal would reduce the FTC'c fiscal 1981 budget from the $77.9 million proposed by the Carter administration to $67.7 million, a cut the agency said would reduce its work force by one-seventh, or 250 employees. The OMB plan proposed a reduction to $59.4 million for fiscal 1982, and by fiscal 1985 the FTC budget would be reduced to $41 million.

Packwood, whose committee has the responsibility of overseeing the FTC, was said by an aide to be "resolute" about his opposition to eliminating the FTC'c antitrust functions. After his meeting with Reagan, Packwood said he had previously told OMB officials that the original FTC budget plan would be a "mistake on principle" and a "mistake politically."

"Small business, that's our constituency," Packwood said yesterday. "From the standpoint of small business in this country, I think the Federal Trade Commission does a better job than the Department of Justice [in antitrust enforcement].

"From the standpoint of big business, I can understand why they would rather have it consolidated in the Department of Justice. But our constituency is the stationery store owner, not General Motors," Packwood said.