Federal workers and retirees covered by Blue Cross-Blue Shield health insurance are due $100 million in refunds in addition to the $280 million rebate already set for them by the giant health insurance company, the General Accounting Office contends in a draft report.

The congressional audit agency's investigation of the refund proposal was undertaken at the request of Rep. Mary Rose Oakar (D-Ohio).

Blue Cross-Blue Shield has proposed refunding $280 million to policyholders and $479 million to the government, based on the way premiums are split between the government and enrollees.

But GAO has tentatively concluded that individual policyholders should get bigger refunds and the refund proposed by Blue Cross-Blue Shield to the government should be cut back.

Adding $100 million to the refunds already proposed for 1.5 million Blue Cross-Blue Shield policyholders would substantially increase the amounts due them.

Premium refunds from half a dozen plans are pending for more than 300,000 federal workers and retirees in the Washington area. The refunds range from less than $20 for postal employes with single-option coverage to $369 for other federal workers with high-option family plans.

More than 80 percent of the workers and retirees who are due refunds belong to the Blue Cross-Blue Shield plan.

An official of Blue Cross-Blue Shield said the company had heard about the GAO conclusions but could not comment until the report is official.

He said the plan would be "delighted" to increase the amount of refunds to individual policyholders, but he cautioned that this could further delay the refunds, which are not expected to be made until January.

An aide to Oakar declined to comment until the report is completed.

In May the company announced that it wanted to make refunds to the government and individual policyholders because of a drop in usage and an increase in the percentage of health bills being paid by the enrollees.

The refunds have been approved by the Reagan administration and the House. The Senate is expected to clear them after the Thanksgiving recess.

Shortly after Blue Cross-Blue Shield made the refund offer, five other federal health plans announced they were planning refunds.

It could not be learned whether the GAO report covers refunds proposed by Aetna, the American Federation of Government Employees, Government Employees Hospital Association, the National Association of Letter Carriers, Government Employees Benefit plan and the Foreign Service plan. About 50,000 policyholders of those plans live in this area.

The refunds have been approved for current employes, but congressional action is necessary before retirees -- who pay the same premiums for the same coverage -- can get refunds as well.

Any recommendaton from the GAO to increase the refunds to policyholders would be nonbinding. But it could cause Congress or the insurance company to reconsider the way the refunds are to be split.

That in turn could cause delays in mailing refund checks.