Pan American World Airways yesterday proposed a new budget fare to major cities in the Far East at half the cost of current economy rates - bringing to transpacific travel the era of cheaper North Atlantic air service launched in the last month.

On Capitol Hill, meanwhile, evidence is growing that opponents of reduced federal controls over the airline industry have been successful in delaying Senate action this year on proposed deregulation legislation.

Lower fares for customers of Pan Am on its services across the Pacific, starting Jan. 15, were proposed to the Civil Aeronautics Board and detailed at a news conference here yesterday. Whether the fare cuts actually take place, however, depends on approval by all the governments involved, and some may not be receptive.

Pan Am executive vice president and board member Dan A. Colussy said the proposed advance-reservation fare would be offered between New York, Los Angeles, San Francisco, Portland, Honolulu and Seattle in this country and Bangkok, Guam, Hong Kong, Manila Okinawa, Osaka, Singapore, Taipei and Tokyo.

Under the plan, one-way budget fares from New York to Tokyo would be $349, a savings of 49 per cent from current economy fares. From West Coast points, the budget fare to Tokyo would be $259, a savings of 48 per cent; from Honolulu, the rate would be $199, for a savings of 51 per cent.

Colussy said Pan Am's decision to bring budget fares to the Pacific is based on the success to date with North Atlantic fare-cutting and the company's hopes of filling now-empty seats on many of its regular flights.

As with the new Atlantic budget fores, customers on Pacific flights would have to call the airline or travel agent at least 21 days in advance of the week they will travel; Pan Am would notify passengers of the exact flight one week before the week of departure.

Colussy said Pan Am was ready to take reservations for the service starting yesterday and hopes to carry 35,000 budget riders in 1978. But he conceded that the fares may be rejected for some of the proposed markets.

In particular, he said approval by Japan "probably will be one of the most difficult ones . . . " Japan and the U.S. recently opened negotiations on a bilateral air service pact but there was little progress by the time the talks were recessed in Tokyo earlier this week.

The Japanese talks are set tentatively to move to Washington in November but the U.S. has met opposition on proposals for sharply reduced fares and liberalized charter rules. If the CAB and President Carter approved the fares but Japan blocks them. Pan Am still intends to introduce them in the other markets, Colussy stated.

Meanwhile, yesterday, legislation to reduce CAB control over U.S. airlines was held up in the Senate Commerce Committee by a long debate over a majority of the committee clearly favors reporting out the bill, which the members have been writing since last summer, it now is considered unlikely that the full Senate could take up the measure before adjournment.

Committee chairman Warren Magnuson (D-Wash.) said in an interview that he "will try to get [the bill] out of committee next Tuesday" but he said he did not expect floor action before the new year.

He called the legislation "a bill of great importance" and indicated he had reservations about some sections, which he declined to identify. "It's much better than it was originally," he said of the measure, which would permit airlines to enter new markets gradually without prior CAB approval.

Proponents of the legislation have expressed concern that what they saw as momentum building in favor of deregulation may be lost if Senate action is delayed until next year.

Union leaders and some industry executives who oppose the administration-backed bill have said a new environment of cheaper fares and competition makes such legislation unneccesary.

In in busy day of developments affecting travelers and the air industry, there also were these developments:

Alitalia announced plans to slash fares between the U.S. and Italy by up to 26 per cent, subject to approval by the Civil Aeronautics Board and President Carter. Unlike similar reduced fares offered by Pan Am, British Airways, Laker Airways and other lines, however, the proposed Alitalia rates would be subject to no limitations.

Umberto Nordio, president of Alitalia, said his firm's entire flying fleet would offer the low fares without such requirements as standby availability or reservations made far in advance. Starting Nov. 20, if approved by the U.S. government, first-class, round-trip service between New York and Rome would be cut to $1,200 from $1,594, and excursion fares would be $460 instead of $565.

McDonnell Douglas Corp., the St. Louis aerospace company, announced a "go ahead" for production of a larger version of the DC-9 twin-jet that the firm says will be "significantly quieter" than jets now in airline service. With 137 passengers, the new wide-cabin planes can be flown 2,000 miles nonstop."

A decision to build the so-called DC-9 "Super 80" came after McDonnell received firm orders or options on 36 of the new jets from Southern Airways, Austrian Airlines and Swissair; the value of 27 firm orders to date exceeds $400 million.

The new McDonnell Douglas plane is the sixth basic model of the twinjet; a total of 914 DC-9s have been ordered to date, of which 861 have been delivered.

Braniff International reported flat profits in the third quarter and record earnings for the first nine months of 1977. Chairman Harding L. Lawrence also said advance bookings are substantially higher than a year ago, leading to optimism that Braniff "will continue to exceed last year's record results."

In the third quarter, Braniff earned $8.27 million compared with $8.2 million in the same period last year, or nues rose to $200 million from $174 million.

Lawrence said the flat performance resulted from lower capital gains, an accounting writeroff in Braniff's hotel subsidiary, ad expenses associated with developing new routes in the Colorado, Oklahoma, Georgia and Florida markets.

Nine-month profits rose to $24.5 million ($1.23 a share) from $17 million (84 cents) last year, and revenues increased to $574 million from $499 million. Passenger traffic was up 7.4 per cent for the nine months and nearly 14 per cent for the first 18 days of October.