Scores of major Virginia retailers, taking advantage of a new law that removes the state's 18 percent interest rate ceiling on credit cards, next month will raise the rates they charge their credit customers to as much as 21.6 percent.

No banks in the state have announced plans to raise rates on MasterCard, Visa or other bank cards above the 18 percent rate, according to a top official at the Bank of Virginia, which runs the state's largest bank credit card system.

The merchants' decisions to boost their interest charges will be effective April 1, when the state's interest rate ceiling is abolished by a law the 1982 General Assembly enacted after a furious lobbying campaign by banking and business interests.

At the time, interest rates nationally had begun to decline from record highs and some legislators expressed hope that the rates would float with the prevailing interest rates. The prime rate, the rate banks charge their most creditworthy corporate customers, has dropped to 10.5 percent and some retailers have been able to arrange recent borrowings as low as 9 percent.

Virginia consumer advocates expressed disappointment yesterday in the new increases, coming at a time of continued decreases in interest rates. Patricia Devlin, deputy director of the Fairfax County consumer affairs office, called the planned increases "a bit excessive given lower interest rates. Everyone was hoping consumers would lead the way out of the recession. To make credit more expensive isn't going to help the economy very much."

Sears credit official John Skinner acknowledged that the company is raising its credit rates in Virginia to 21 percent, the interest rate that Sears charges in at least 22 other states. Skinner said the increase was necessary because the company has suffered major losses in its credit operations in recent years--$83 million in 1981 and $25 million in 1982.

"We haven't increased the interest rate for about 20 years," Skinner said. "It was 18 percent in the 1960s when the prime rate was only 5 percent."

Not all retail stores in Virginia are raising their rates. Hechinger and Raleigh's have said they plan to hold rates to 18 percent for the moment. But the Raleigh's spokesman said, "This is only a short-term decision, but it will give us a slight competitive advantage on our charge accounts."

Both continue to charge Maryland and District customers 18 percent. Maryland last year allowed its credit card rates to rise to a maximum 24 percent. The District imposes a maximum 18 percent interest limit.

Most of the Virginia stores boosting their rates indicated yesterday they will be charging 21 or 21.6 percent interest on an annual basis, the prevailing interest rate for credit card accounts in about half the states.

Although some consumer groups had predicted that bankers would take advantage of the new law and "throw consumers to the wolves," David K. Hunt, a senior vice president of the Bank of Virginia, said he didn't expect any banks would be raising rates in the near future. In fact, the higher interest rates charged by the merchants, Hunt said, "will give us a little competitive edge."

Retail store officials pointed out, however, that most holders of bank credit cards now pay a $15-to-$20 annual membership fee, approved by the legislature in 1980, and that consumers usually pay interest on bank credit card purchases from the moment of the sale, unlike the interest on retail sales, which may not begin until 30 to 59 days after the sale.

The interest charge rates for both banks and retail stores are determined by the residence of the consumer, not the location of the store or bank credit card company. In April, for instance, a Virginia resident will pay 21 percent interest on purchases at any Sears store, even from a store in the District where the 18 percent ceiling is in effect.