Bowing to pressure from Congress, the General Services Administration has delayed for 30 days final bidding on a controversial $4.5 billion telecommunications network for the federal government.

The postponement disclosed yesterday is the latest in a series of Byzantine maneuvers over the contract, one of the largest ever put out for bids by the government.

If built, the proposed government network, known as FTS-2000, would be the largest private phone system in the world. The government's present phone system is considered expensive to operate and outdated.

The contract has provoked intense lobbying by the bidding companies as well as tension between Congress and GSA, and between GSA and other federal agencies.

Sources involved in the bidding said yesterday that the latest postponement -- the third since the procurement began -- has renewed concern among the bidders about whether the contract will ever be awarded. Final bids were originally due June 30; GSA said yesterday that the bids will now be received Sept. 30.

A GSA spokesman said the delay was ordered "in direct response" to a General Accounting Office study commissioned by Sen. John Glenn (D-Ohio), chairman of the Senate governmental affairs committee.

The GAO study, the second of FTS-2000 undertaken by the office, criticized GSA's approach to the procurement, saying the agency had failed to consider alternatives to its winner-take-all approach to the contract and that it had done inadequate long-range planning.

But whereas the GAO had recommended in May that the FTS-2000 bid be scrapped, its report released yesterday conceded that "there are some short-term advantages to allowing FTS-2000 to proceed."

In a letter to the GAO released yesterday, GSA chief Terence C. Golden responded to the accounting office's study by agreeing to a 30-day delay so that some of GAO's recommendations could be implemented. Golden said in the letter that GSA would clarify its bidding rules so that the government would clearly have the right to escape its FTS contract after spending about $450 million, which a GSA spokesman estimated would take four years.

But some sources involved suggested another purpose of the delay was to buy time for a possible political compromise between GSA and Rep. Jack Brooks (D-Tex.), chairman of the House government operations committee and a critic of GSA's approach to the FTS contract.

Brooks has said that GSA's winner-take-all approach has not created adequate competition among bidding companies for the government's lucrative phone business. Congressional sources said yesterday that the GAO's most recent recommendations probably will not alleviate Brooks' concerns.

Only two groups of bidding companies are left in the competition -- one headed by American Telephone & Telegraph Co., the other by Bethesda-based Martin Marietta Corp., which has allied itself with MCI Communications Corp. of Washington. A third consortium led by U.S. Sprint Communications Co. dropped out when Electronic Data Systems Corp. withdrew from the group, citing government uncertainties and delays in the bidding process.

"Our view is basically one of support for this delay if it helps to resolve the concerns that have been raised by Congress about the contract," said Edie Herman, an AT&T spokeswoman.

David Wonderling, a Martin Marietta spokesman, said that the impact of procurement changes proposed by GAO and of the delay in bidding was under review. "We're trying to evaluate the likelihood that this will be the last delay," Wonderling said.

The two bidding consortia have invested much time and money in their proposals, which they had expected to submit to GSA next week. Sources familiar with Martin Marietta's bid said it exceeds 3,000 pages.