In response to this week's major move by the Civil Aeronautics Board to deregulate air fares, Braniff International announced yesterday it will raise domestic fares as much as 50 percent by July 1.

Braniff said it will charge the maximum fare the CAB allowed in two stages. Beginning May 23, Braniff said it will raise fares by 30 percent over the current allowable fare on routes under 400 miles, Braniff said it will raise fares by 20 percent.

Effective July 1, Braniff will raise fares again so that the fares on routes under 400 miles, Braniff will raise fares so they are a total of 30 percent above the May 1 ceiling.

The increases could be somewhat less on some routes because the airlines already had increased fares between 5 and 10 percent allowed under CAB rules and Braniff, as others, had used that provision to boost fares.

The CAB action this week approved a new three-tiered system of fare flexibility for the airline industry.Effective immediately, the plan allowed airlines to raise fares, without CAB approval, an unlimited amount for routes of 200 miles or less; 50 percent above the base level for routes up to 400 miles; and 30 percent above the base for routes over 400 miles.

Braniff said the fare increases would permit a stabilization of fares -- until at least November -- and at the same time help to offset the dramatic increases in fuel prices the airlines have suffered. "Braniff believes that traffic will not be affected adversely by higher fares because air travel will still remain a considerable bargain," a company statement said.

It's very doubtful that other airlines will follow Braniff's lead and use the fare flexibility provisions to the maximum extent; many airline officials are worried that higher fares are beginning to hurt customer travel plans.

Several airlines, including American, United and Trans World, have indicated they will make increases of about 4 or 5 percent at the most as a result of the new CAB rules.

Jon Ash, TWA vice president for government affairs, said yesterday TWA planned a 5 percent increase across the board effective June 1 with no mileage differentiation, even though it is allowed by the CAB. Ash said the increase would not apply to its services between New York and the West Coast and would be accompanied by one or two more general fare increases as well.

In another airline development, World Airways yesterday unveiled summer fares on its new air service between Washington and London that are about 40 percent lower than comparable competitive fares.

The service from Baltimore-Washington International Airport, three times a week, includes a one-hour stop in each direction in Boston, World's newly authorized gateway from the United States to London's Gatwick Airport. The other our days a week, World's transatlantic service will begin in Newark.

Although it will begin service on June 1 with an even lower introductory fare, World set its regular summer fare for a reserved coach seat with meal service at $300 each way. In comparison, a regular coach seat on Pan American World Airways this summer is $470 each way, with a fare increase likely to go into effect June 1.

World's regular round-trip coach fare is even lower than Pan Am's advanced-purchase excursion fare of $660 which requires the traveler to buy a ticket at least 21 days in advance and stay at least seven days. Pan Am's advertised summer standby fare is $497 round trip.