In a slap at President Reagan's consumer adviser, Virginia H. Knauer, the administration's chief regulatory adviser has made it clear that the White House does not support the Federal Trade Commission's used-car rule.

Christopher DeMuth, the head of the Office of Management and Budget's Office for Information and Regulatory Affairs, told Congress this week that Knauer's statement in support of the rule was "her own personal opinion on the rule and was not an administration position."

DeMuth's comments, made in a letter to a congressmen who opposes the rule, comes just as Congress is preparing to vote to overturn the FTC regulation.

Although Knauer supported the rule on the ground that it would protect consumers from disreputable auto dealers who make misrepresentations to prospective buyers, DeMuth said he was concerned that the rule would impose a serious "paperwork burden" on small businessmen.

Issued last August, the rule would require used-car dealers to disclose more information to consumers about the cars they sell. A one-page "Used-Car Buyers Guide" would have to be attached to each car, telling consumers about their warranty rights and about any known defects about the car.

The bill has been vigorously opposed by the used-car industry throughout the FTC's lengthy rule-making procedures. Now the industry is urging Congress to veto the bill. Under legislation passed last year, Congress can overturn any FTC rule with a majority vote of both houses. Both the Senate and House Commerce Committees are scheduled to vote on a resolution of disapproval today and congressional aides predict a veto could well pass Congress before it adjourns for the Christmas recess.

Last week, the rule's supporters had garnered some hope from Knauer, who called it a "reasonable, straightforward and uncomplicated" approach that "provides significant benefits to consumers."

But DeMuth's criticisms have now dashed their hopes.