The civil Aeronautics Board yesterday approved increases ranging up to 25 per cent of most fares for flights across the North Atlantic.

The board refused, however, to go along with proposed increases in the normal economy fares to Europe which it said already were "significantly in excess of cost" or with a proposed boost in fares for the Miami-London route.

The increases were proposed by members of the International Air Transport Association to be effective April 1, but the board said it would not allow increases to go into effect before June 1 to protect travelers who had already made spring travel plans for the higherst fares.

The board also approved a change from the three-season fare structure traditional across the North Atlantic to a two-season structure. In the past there was a peak seasons on either side (April-May and September-October), and winter (November through March). Winter fares used to be the cheapest but the airlines last year challenged a CAB order requiring them to drop their shoulder rates to the winter levels and won. Yesterday, the CAB approved a two-season structure: peak, from May 15 to September 15, and basic - the rest of the year. However, the board also said it would allow a "peak of the peak" surcharge also.

As examples of what yesterday's board decision means to consumers, using a New York-London round-trip; a first class ticket will rise 5 per cent from $1,250 to $1,312. There will be no change in the 14-21 day excursion in the peak season will be 11.4 per cent higher - $587.

The promotional fares will rise as well. The advance purchase excursion fare, for instance, will rise 7.7 per cent to $440 this summer from last year's $440 this summer from last year's $410. The board also shortened the advance purchase requirement for the APEX fares to 45 days from 60 and shortened the length of stay required to 14 days from 22 beginning in October.

In other international flight matters, the board.

Granted National Airlines the authority to fly between Miami and Paris for 90 days beginning June 22, giving the airline an alternate route to Europe in case the United States and the United Kingdom don't reach a new air transportation agreement by then. National currently flies a Miami-London route.

The U.S. and U.K. currently are trying to work out an agreement to replace the existing pact which will expire on that date. Last June the British gave notice they wanted to terminate the 30-year-old agreement governing air relations between the two and are seeking a new one asuring their carriers of carrying half the traffic between the two countries.

Took an initial step in reactivating consideration of the UK's Laker Airways proposal to offer low-cost air serviceacross the North Atlantic.