Delta Air Lines and Northwest Airlines would gain transatlantic routes for the first time under a tenative order issued yesterday by the Civil Aeronautics Board.

In another major decision, the five member agency broke precedent for federal regulatory bodies by signaling approval of faires set on the basis of expected costs.

The practical result of the so-called "anticipatory" rate making process will be higher domestic fares for U.S. passengers starting later this month. Eleven airlines plan to increase fares 1.8 per cent to 2 per cent on various dates between Sept. 15 and Oct. 10.

In granting new authorities for transatlantic service, the CAB added 13 U.S. cities as "gateways" to Europe - in part to implement parts of a new accord between the U.S. and Britain on service between the two nations.

The agency's decisions yesterday amounted to directions to the CAB staff to draw up final orders, on which the agency will vote formally in the next two weeks.

As with all international route award cases, the CAB recommendations are submitted to President Carter for final action. White Hose spokesmen have said they expect Carter to announce his decisions before Nov. 1.

Specifically, the CAB indicated it will approve in its final recommendations:

Non-stop authority between Atlanta and London, the first ever, by Delta. Under the U.S.-British accord, an American airline will have exclusive non-stop rights between Atlanta and London for three years. Pan American World Airways had fought strongly to gain the new route.

Initial transatlantic authority for Northwest Orient Airlines between New York and Copenhagen and between Minneapolis-St. Paul and Norway, Sweden, Denmark and Finland. On the Scandinavian routes, Northwest would replace Pan Am, a decision that the international carrier "did not fight with vigor," a spokesman noted yesterday.

Pan American service to London from new gateway cities of Dallas and Ft. Worth, rejecting a Braniff International bid for the new route, Pan Am expects to offer daily Boeing 747 flights from Houston to Dallas-Ft. Worth, London and Frankfurt starting this fall, if approved by Carter.

National Airlines nonstop service from Miami to Paris. The service began June 22 under a temporary order, but this would make it permanent. National also was awarded nonstop authority from the new gateway cities of Tampa and New Orleans and Paris. A National spokesman last night did not know whether flights from those cities would stop at Miami en route.

Permanent authority for Pan Am to fly nonstop from Miami to Madrid, a service started under temporary authority this summer, and expected nonstop authority for TWA between London and Cleveland, St. Louis, Denver, Kansas City and Minneapolis-St. Paul.

The CAB's action yesterday, in giving the hotly-contested transatlantic routes to established carriers, rebuffed a Justice Department proposal that low-fare carriers should be considered. But the regulatory agency was undecided on whether the new route awards would be effective for three or five years and asked its staff for recommendations.

Although the agency said granting authority to a low-fare carrier on an experimental basis would be impractical, a tentative decision late last week would permit six scheduled airlines to begin offering cut-rate fares Sept. 26.

In its domestic fare decision yesterday, the CAB members supported recommendations of chairman Alfred E. Kahn. The significant change in policy will permit airlines to forecast expected costs in seeking higher rates. The net result should be fewer fare hikes.

Although the airlines' revenues during initial months under such rate increases would be higher than necessary, there would be a deficiency in later months, an agency spokesman said.

During 1977 alone, domestic airlines have increased fares four times to date - each time by small amounts.