In an action that promises to intensify the current low-fare competition in the airline industry, the Civil Aeronautics Board yesterday voted to significantly liberalize its rules governing charter flights.

The modifications are designed to enable charter airlines to compete more effectively with the discount fares now being offered by scheduled airlines on domestic and international flights.

Under yesterday's action, which becomes effective at least for domestic flights - as soon as the board's formal order is written:

The advance purchase period for popular Advance Booking Charters (ABCs) will be reduced to 15 days from the current 45 days for most European destinations and to 30 days for all other destinations.

A tour operator will be able to sell 15 per cent of the seats of an ABC group after the advance-purchase time. Added to the board's current policy of allowing a charter operator to substitute up to 15 per cent of the passengers after the advance-purchase period, this change means late-booking passengers could comprise up to 30 per cent of the charter group. It is designed to allow the charter carrier more freedom to compete for the short-notice traveler.

The minimum group size for ABC's and One-stop-inclusive Tour Charters (OTCs) will be reduced from 40 to 20. The added flexibility provided by allowing a tour operator to schedule more numerous but smaller groups on the airline is designed to reduce their costs and lessen the risk of flight cancellations.

Minimum-stay requirements on the ABCs and OTCs will be eliminated. Current CAB rules require a traveler to stay at least seven days in Europe on an ABC and seven days on international OTCs, and at least four days on North American OTCs.

Although the new rules affecting international charters become effective almost immediately along with the domestic rules as far as the CAB is concerned, charter operations to most European destinations aren't likely to change immediately.

International charters generally have to comply with both CAB rules and acceptance conditions set by the country of destination.

For instance., most European aviation authorities currently allow U.S. ABCs to land in their countries only if the charters have met their much more stringent rules, such as a 14-day minimum stay requirement.

The U.S. has been trying to persuade European governments to withdraw those restrictions, but has met with resistance. A number of countries already have said flatly that they would be unwilling to accept the ABCs under the new rules, a CAB ficial said yesterday.

There are tow exceptions. Last month, Belgium agreed to a new air agreement with the U.S. which allows the U.S. to set the rules governing charters to and from Belgium, so the new charter rules may well increase charter traffic to Belgium. In addition, Yugoslavia has a similar agreement with the U.S.

The CAB proposed liberalized its charter rules this fall because of changes in both the domestic and international aviation pictures.

On the domestic front, the charter carriers complained that deep discound eliminate them from that for-especially on transcontinental routes, could eliminate them from the formerly lucrative market. Also, the entry of Laker Airways' low-fare Skytrain air passenger service between New York and London had elicited a low-fare response from the scheduled carriers that some worried could drive the charters out of business altogether.