A federal grand jury has begun an investigation into allegations of payoffs made by Computer Sciences Corp. to obtain a General Services Administration contract -- which has amounted to $100 million -- to provide government agencies with computer services.

According to the information presented to the grand jury, by Justice Department prosecutors here, a Computer Sciences official based at the giant corporation's EI Segundo, Calif., headquarters made cash payments to high-ranking GSA officials responsible for awarding the contract to the firm.

In addition, the grand jury is probing findings by GAS auditors and investigators that GSA overpaid Computer Sciences more than $300,000 for the services of computer consultants whose qualifications were falsified to put them into higher pay brackets.

By overstating the number of years of experience and education the consultants had, some were paid as high as $45 an hour when they had no relevant experience, and others were paid at $25 an hour when they should have been paid only $16, according to GSA audit findings.

A Computer Sciences spokesman yesterday denied that any payoffs were made to obtain the GSA computer contract.

"The National Teleprocessing Services Contract was awarded to Computer Sciences in 1972 after a lengthy and highly competitive procurement procedure, administered by a government interagency evaluation committee. Computer Sciences participated in this procurement in a completely ethical manner," the spokesman said.

He said the company has cooperated "fully" with GSA its in its investigation of the contract and noted the company has not been told of any grand jury investigation. A GSA audit report on Computer Sciences' consultants, the spokesman said, stated there was "no basis for any criminal investigation" of the overpayments.

The grand jury, which began hearing evidence in the case a week ago, is being supervised by William S. Lynch, chief of a new Justice Department task force formed to investigate GSA abuses.

The Computer Sciences case is the first to be presented by the new task force to a grand jury.

Besides providing federal workers with office space and supplies, GSA awards contracts to provide them with computer services. No one knows exactly how much money this entails. However, the General Accounting Office. the audit arm of Congress, estimates it amounts to $10 billion a year.

As in most other purchases by GSA, computer contracts are awarded without sealed, competitive bids, allowing the agency substantial room for exercising judgment on which firm to select.

GSA hired Computer Sciences to provide consultants and services to GSA and to advise on the operation of a remote terminal system -- called Infonet -- for 7,500 government users. The system allows government workers in various agencies to use a centrally-located computer from terminals nals in their offices.

Since the award was made in 1972, GSA and other government users have paid Computer Sciences nearly $100 million for services.

Included in that figure is more than $300,000 GSA auditors and investigators contend GSA overpaid.

The consultants were employed by Computer Sciences and two of its subcontractors. The records of one of the subcontractors, Icarus Corp. of Rockville, have been subpoenaed by the grand jury.

Icarus' president, Herbert Blecker, did not return a reporter's telephone calls yesterday.

Besides the Computer Sciences investigation, the Justice Department task force is probing building lease awards made by former GSA administrator Arthur F. Sampson to the Buzzard Point building, owned by Dr. Laszlo N. Tauber and his partners, and to buildings owned by Peter, Paul and Louis Pomponio, as well as furniture contracts awarded to Art Metal USA Inc. of Newark, N.J.