A group of bankers yesterday accused the Big Three auto makers of illegally offering cut-rate financing on new-car loans.

The sales incentive campaigns, such as Chrysler Credit's 5.5 percent financing program and the 5.9 percent loans offered by General Motors Acceptance Corp. and Ford Credit, are subsidized by the corporate parents of those captive finance organizations.

Such subsidized loans are illegal because they undermine legitimate competition in the car loan market, according to a complaint filed by the Consumer Bankers Association with attorneys general in 13 states.

CBA, which is based in Arlington, represents 670 federally insured banks, thrifts and credit unions nationwide.

"The Consumer Bankers Association is very concerned about such subvention plans because they enable the financing captives of automobile manufacturers to monopolize the consumer financing of their respective automobiles," the CBA complaint said.

"Not only is such monopolistic conduct illegal, but it is extremely imprudent for many reasons of public policy," the CBA complaint said.

The complaint is contained in letters mailed to the attorneys general of Arkansas, California, Colorado, Florida, Hawaii, Idaho, Louisiana, Nevada, New Mexico, Ohio, Tennessee, Utah and Wisconsin.

According to CBA, those states have laws forbidding automobile manufacturers to pay subsidies to lenders "if the effect of such subsidy may be to lessen competition or tend to create a monopoly."

Representatives of the nation's big three auto makers, General Motors Corp., Ford Motor Co. and Chrysler Corp., yesterday condemned the CBA's petition. They were joined in that attack by an unusual ally, Joan Claybrook, president of Public Citizen, a consumers' action lobby that routinely has taken issue with the car companies. The group has its headquarters in Washington.

"I'm with the automobile manufacturers on this one," Claybrook said. "Consumers ought to have competing sources of auto loans. Why should the banks have a monopoly?"

Claybrook said the widespread use of sales-incentive campaigns by the nation's auto makers "is competition in its best form."

"To complain that competition is a monopoly is really a unique argument. I have to give the banks credit for creativity on this one," Claybrook said.

The low-cost financing programs, which are expected to stay in effect, in one form or another, throughout calendar 1986, are "a reflection of competition among automobile companies," said John P. Danforth of Golembe Associates, a bank consulting firm with headquarters in Washington.

"It strikes me as untrue" that the sales-incentive programs "reflect attempts by the captive finance companies to create a monopoly," Danforth said.

But the CBA, citing an example in its complaint, said, "The net effect of several or more months of subvention has been a great reduction in the amount of new-car financing done by banks, et cetera, and the creation of a new monopoly of the financing of GM-made cars by GMAC."

The bankers association offered no specific figures to support its argument. But statistics from the Federal Reserve Board show commercial bankers are losing their share of the car-loan business.

As of January 1986, commercial banks held 47 percent of the $293 billion outstanding in auto loans. Financial service companies such as Ford Motor Credit held 36 percent of the auto loans, and credit unions held 18 percent.

In comparison, in January 1977, commercial banks held 59.8 percent of the $82.9 billion then outstanding in auto loans, financial services companies held 18.4 percent and credit unions held 21.8 percent, according to Federal Reserve figures.

In another sign of the changes in car financing, Sears said yesterday it will begin offering auto loans on its Discover Card. The card -- good at Sears stores and many other retailers -- will be used for auto financing in California, Colorado, Florida, Georgia, Illinois, Indiana, Kansas, New York, Ohio, Pennsylvania and Wisconsin.