MCI Communications Corp., still smarting over being shut out of the federal government's huge FTS-2000 telecommunications system two years ago, yesterday announced what amounts to a rival network.

MCI urged federal agencies to use a new discount government service it unveiled, saying it could save taxpayers $100 million a year.

It accused American Telephone & Telegraph Co. and US Sprint Communications Co., which won the FTS contract, of giving corporate customers better deals than the government gets.

AT&T and Sprint shrugged off the move, saying their commercial and government rates are similar. "This action appears to be just another example of MCI's ongoing effort to undermine legitimate, federally procured contracts," AT&T spokesman Jim McGann said. MCI efforts to win agencies away from FTS "undermines Congress's intent and subverts the law," he said.

Gary Forsee, president of Sprint's business services group, said MCI was offering a limited service without full customer backup.

Precise comparisons of prices are hard to make because of volume discounts and complex formulas that take into account the time of day and distance. But analysts said MCI was playing to pricing dissatisfaction that exists in some federal agencies over the newly installed network.

"MCI is stirring up the FTS-2000 pot," said Gregory P. Cline of Information Strategies Group, a Vienna-based research group.

As a subcontractor to Martin Marietta Corp. of Bethesda, MCI bid for a piece of FTS-2000 in 1988 but lost. It later filed a number of unsuccessful protests to the contract award.

FTS-2000 was created by Congress to bring modern telecommunications services to federal offices and save money by lumping them together to achieve volume discounts. But some agencies have found they can get better deals on their own.