Freddie Laker, the British businessman who is battling the major airlines, won a major battle for his cut rate trans-Atlantic fare today.

The Civil Aviation Authority here ruled that Laker's $239, round trip flights between London and New York may use convenient Gatwick Airport, and dropped its demand that his no-frills service operate from remote and rural Stansted.

In addition, the Authority said Laker's daily DC-10 may carry a full load of 345 passengers each way throughout the year. Up to now, and at the urging of Laker's large competitors, his cut-rate flights were to be limited to 189 passengers in the slower winter season.

"We're delighted. We all are," an ebullient Laker said. If Laker had been forced to fly from Standsted, 35 miles and two hours from central London he would have been at a competitive disadvantage against Pan Am, TWA, British Airways and other major airlines seeking to match Laker's fares. British Airways sent a top official to plead with the Authority to keep Laker's airport and passenger curbs, but the airline was turned down.Gatwich is only 40 minutes by train from the heart of London.

The biggest beneficiaries of today's order are likely to be what the trade calls the "knabsack and guitar brigade" college students from Britain and America whose slim budgets limits their flying.

Laker passengers will line up as early as 4 a.m. each day, and the first 345 will get seats. The $239 fare for the round trip is considerably lower than the major airlines "economy fare, which range from $350 to close to $700.

Pan Am and the other members of the International Air Transport Association have responded with $256 round-trip stad-by service. If there are empty seats on their scheduled flight, waiting without reservations passengers will be able to take them at the low price in some cases.

The major had wanted another plan, a $256 reserved flight for those booking these weeks in advance, permitting the airline to choose the day of departure. That was knocked out by the Civil Aeronautics Board in Washington Friday.

The Justice Department had attacked it as a predatory device to cripple Laker and charter flights, and the CAB prohibited it. Government officials apparently feared that the majors would run this "budget" plan until Laker and the charter carriers were driven from the air, thus allowing the major scheduled carriers to end cut-rate services.

In London, the Civil Aviation Authority said it was eliminating the airport and passenger curbs on Laker because its service is "a valuable experiment directed toward the broadening of the air transport market and the improvement of consumer choice and satisfaction."

But, the Authority, attached one new condition: It forbade Laker to sell tickets and check in baggage at Gatwick, saying it feared that crowds of waiting passenger would disrupt operations. Laker Airways has no ticket offices, however, one reason it can slash prices. The airline says it is confident that it will get this order reversed after an appeal. Laker himself tells all comers that the Authority loves him, that he is regarded as a symbol of government concern for consumers.