Major transatlantic airlines have agreed to offer a cut-rate $256 New York-London round-trip fare beginning Sept. 15 to compete with a no-frills scheduled airline fare to be started by a British charter carrier next month.

The International Air Transport Association said today that six member airlines, including Pan American World Airways and Trans World Airlines, will offer the $256 fare - a saving of up to $370 from normal economy prices - under two plans with various limitations. A one-way fare would be $146.

The British carrier, Laker Airways, plans to start a no-reservation "Skytrain" service without meals for $236 round-trip or $135 one-way.

Under one plan, the passenger cannot pick the day he wants to fly, only the week, and must purchase the tickets at least three weeks in advance. The airlines would inform the passenger of the exact date and time of flight, 10 days or less before the week of departure.

The other plan is a standby arrangement - with no advance booking or guarantee of getting a seat. Seats would be assigned only after 4 a.m. on the day of the flight and could not be bought with three hours of the flight time in New York, or within two hours in London. There will be a limit of 2,900 low-cost seats available each week in either direction.

Tickets on Laker's Skytrain will be available six hours before each flight.

Unlike the Laker flights, which will go to London's distant Stansted Airport, the low-fares on the IATA carriers will be available on their regular flights to Heathrow, closer to the city. Also, meals will be included in the ticket price; they are not on the Skytrain.

The proposed fares, which would run until March 15, 1978, must be approved by some IATA carriers who were not at the IATA meeting in Geneva late last week when agreement was reached, as well as by the governments of the airlines involved, but no trouble with approval is expected.

Aside from the lower transatlantic charter fares, the lowest fare available to scheduled-airline travelers between New York and London during the off-peak season (Sept. 15 to May 14) is $350 roundtrip, for a 22- to 45-day stay with a requirement that the ticket of the flight. A normal economy round-trip ticket is $626; a 14-21 day excursion round-trip ticket is $541.

IATA also said that some fares between certain European and U.S. cities will also be lowered on the 22- to 45-day advance-purchase excursion (APEX) fares beginning Oct. 1.

Specific fares for these cities will not be available until a technical working group completes the details at a meeting that begins in Geneva today.

Six airlines will operate the low-cost budget and standby fares: Pan Am, TWA, British Airways, and three airlines that fly between their own countries and New York via London - Air India, Iran Air and Israel's El Al.

If the airlines had failed to reach agreement on the new fares, an "open-rate" situation would have prevailed, with each airline setting its own fares to try to combat Laker's Skytrain. "The revised fares package will remove much of the present confusion and speculation on these important routes, and will offer the traveling public, the governments and the airlines the necessary stability and order," an IATA announcement said.

According to sources close to the negotiations, British Airways had insisted on a cut in the APEX fare between London and New York as a condition for accepting the budget and standby fares.

Continent European airlines, in turn, said they could not accept an APEX reduction on the London-New York route only because this would put them at a competitive disadvantage, so they argued for similar reductions - to be worked out - on flights between the United States and other major European cities.