Gov. Charles S. Robb today made Virginia the sixth state to impose a moratorium on the controversial cutoff of Social Security benefit payments to thousands of disabled persons.

In a letter to U.S. Health and Human Services Secretary Margaret Heckler, Robb said he was taking the action "to protect disabled Virginians" who might qualify for continued benefits under legislation now before Congress.

"No terminations will be effected until such time as appropriate reforms are made," Robb said. "I withheld taking this step in hopes that some action might be taken at the federal level." Robb said the review process itself will continue.

Since the process began in March 1981, according to state officials, about 14,000 cases have been considered and 6,075 persons have been cut from the program.

Robb's action today came several weeks after Rep. Norman Sisisky, a Southside Virginia Democrat, urged him to suspend the cutoffs. Sisisky is cosponsoring legislation to enact safeguards in the program.

A week ago, Robb said he was not certain he had the authority to stop the cutoffs. A spokesman said yesterday the moratorium was approved by state Attorney General Gerald L. Baliles.

In 1980, Congress ordered the Social Security Administration and the states to conduct a complete review of disability payments and enacted tighter standards for qualifying. The review was to have begun in January 1982, but the Reagan administration moved it up to March 1981.

The cutoffs have sparked nationwide criticisms that needy persons have been denied benefits unfairly.

According to state and federal officials, similar moratoriums on cutoffs already are in effect in North Carolina, West Virginia, Alabama, Georgia and New York.