The Federal Trade Commission inched toward further regulation of used-car sales yesterday, as commission members vehemently denied that they were slowing their efforts in response to congressional pressures.

The used-car regulation was adopted "tentatively" by the commission with the provision that the rule be available for continued public evaluation and comment. Three commissioners voted for that move, as FTC members Robert Pitofsky and David Clanton did not vote.

But commission members reacted bitterly to a report in yesterday's Washington Post, based on interviews and confidential FTC memos, which suggested that the commission members and staff are worried about the used-car rule in light of the ongoing criticism of the agency on Capitol Hill.

FTC Chairman Michael Pertschuk called the disclosure of the internal documents "as serious an effort to undermine the commission's decision-making process as the industry lobbying we decry as improper."

Pertschuk said there is "no evidence" that any FTC decisions "bore the slightest evidence of being politically motivated."

"Not only was the leakage a violation of commission regulations, it did a disservice to the staff and commission."

FTC member Robert Pitofsky, whose memo raising questions about the used-car rule was disclosed, said he is concerned "because some people on the staff would like to forget their own findings."

The decision to delay final approval of the regulation was due largely to several commission members, including Pitofsky, feeling the record behind the used-car rule wasn't complete.

FTC General Counsel Michael Sohn said that "as a matter of procedure and discretion, I don't think it's the best way to make the rule."

In essense, under the proposal, dealers would be required to check off "OK," "Not OK," or "We Don't Know" on a sticker on used cars. The plan, which has been revised several times since the matter came up in 1976, would permit dealers to use any reasonable testing procedure as long as the system met inspection standards set out by the commission.

Pitofsky, in particular, wondered during yesterday's meeting whether the rule would be effective. "Of course it doesn't cure everything," Pitofsky said. "My concern is that it doesn't cure anything."

The rule's most outspoken proponent, FTC member Paul Rand Dixon, said it is time for the commission to "fish or cut bait."