Federal banking regulators have ordered the repayment of an $1.8 million insider loan granted by a California bank partially controlled by presidential confidant and millionaire car dealer Holmes Tuttle.

The beneficiary of the 1978 loan was Tuttle's son, Robert, 37, who used it to purchase stock in a Tucson, Ariz., bank. The collateral for the loan was the stock. The loan was guaranteed by the elder Tuttle and went unpaid for two years while $300,000 in upaid interest also accured.

Robert Tuttle said yesterday in a telephone interview that the principal had been repaid and the outstanding interest refinanced through a "private source." He defended the loan he received and said the action of federal bank officials represented a "philosophical disagreement to that approach to banking." But, he added, "this is a case where we were willing to compromise."

Holmes Tuttle is a longtime director and minority stockholder of the Community Bank of Huntington Park, Calif. The bank, whose customers' deposits are insured and, therefore, regulated by a federal agency, has been seeking to establish a new branch.

The three-member board of the Federal Deposit Insurance Corp., composed of its chairman, director and the acting comptroller of the currency, issued the order Monday as a condition of granting the branch banking license.

Robert Tuttle said the branch, already constructed, opened yesterday after the bank complied with the FDIC order.

James L. Sexton, chief deputy director for bank supervision at the FDIC, said yesterday that "we approved the branch with that condition in it."

The order itself states that, "Prior to . . . the establishment of the branch, the certain $1.8 million loan which Community made to facilitate the purchase of stock of another financial institution by a certain individual shall along with interest . . . be paid in full without recourse to Community Bank." The "certain individual" mentioned in the order was Robert Tuttle, Sexton said.

Sexton would not comment on the reason for requiring repayment, but it was clear that the condition sprang from the fact that the loan to the younger Tuttle had been criticized in the 1980 FDIC examination of the bank's loan portfolio.

A criticized loan is one that represents a potential loss to the bank due to deficiencies in repayment, collateral or other security pledged in the event of default.

Holmes Tuttle was reported by his office to be recovering from surgery and not available for comment, but the Los Angeles Times reported that Tuttle called critical FDIC examination report "a bunch of crap."

Tuttle is a member of the ad hoc group of businessmen and political advisers who came to be known as Ronald Reagan's "kitchen cabinent" during the 1980 presidential campaign.