Relief may be on the way for the hundreds, perhaps thousands, of American travelers stranded in London waiting for seats at discount prices on flights home.

In the hopes of increasing the availability of lowfare seats immediately, the Civil Aeronautics Board yesterday waived its charter regulations for the next 10 days to allow the stranded stand-by passengers in England to return home on charter flights.

The board's action is designed to allow airlines operating charters with empty seats to fill them with the standby passengers, and to allow the formation of new charters just to bring the would-be travelers home without regard to the usual round-trip, advance purchase, and trip duration rules.

The emergency blanket waiver covers all airlines - foreign and domestic - with either scheduled or charter authority to fly between the United States and United Kingdom.

The board acted on reports from London of many, many travelers stranded at Heathrow and Gatwick airports hoping to obtain passage home on scheduled flights using discount fares. Most of the travelers probably flew to Europe on the Laker Skytrain or as stand-by passengers on scheduled flights with relative ease and expected to return the same way, the board noted.

Because this is the busiest time of the transatlantic summer season and more people than ever are traveling, loads on transatlantic flights are very high, leaving few seats available for standbys, the board said. The situation has caused a hardshp on those waiting, many with limited funds, and on the other air travelers who face a good deal of airport congestion, the board said in explaining its action.

The airlines that want to take advantage of the board's waiver still will need special authority from the British government, as well as a CAB waiver of the requirements of the U.S.-U.K. bilateral agreement.

Because the CAB is limited by the bilateral to granting waivers to only 70 fights a year (ending March 31, 1979, the board said it would take into consideration the number of seats an airline would make available to stand-by passengers when granting the waivers. The waivers are to be made on an ad hoc basis by the board's bureau of pricing and domestic aviation.

The British Government acted this week to loosen its regulations to allow transatlantic carriers to use up their standby quotas through October in an effort to relieve the crush of the airports. But one problem appears to be the paucity of available seats. Allotments of stand-by seats are for use at the airlines' discretion, and they use them only if they are unable to sell seats at regular fares. Most of the seats are being filled and with fullfare customers or those who signed up for the advance-purchase excursion fares.

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In another airline development, Federal Express Corp., a successful Memphis-based firm specializing in express air cargo service asked the CAB for new authority to operate passenger service between Chicago's Mid way Airport and 17 other cities.

The board, which recently authorized new services between Midway and six cities, is considering expanded Midway service in a new proceeding.