Laker Airways yesterday asked the British government to loosen some of the restrictions that have been placed on its low-fare "Skytrain" air service between New York and London.

The no-frills, no-reservations flights across the Atlantic - at the round-trip fare of $236 - are scheduled to begin Sept. 26.

Laker's petition to the Civil Aviation Authority followed an announcement by the International Air Transport Association that is member airlines - including Pan America World Airways, Trans World Airlines, and British Airways - had agreed to offer a cut-rate $256 New York-London round-trip fare on their regularly scheduled flights to compete with Laker's Skytrain.

"We started the low-fare revolution on the Atlantic," Freddie Laker, chairman of Laker Airways, complained yesterday. "For six years, the IATA airlines have frustrated every attempt by Laker to introduce Skytrain.

"Now they themselves are claiming a breakthrough."

Laker askked the British government to eliminate the restrictions it has placed on the frequency and seating capacity of the Skytrain, to increase the free baggage allowance from 33 to 44 pounds, to allow travel agents to sell Skytrain tickets, to allow seat allocations beginning at 4 a.m. on each day of departure, and to allow the Skytrain to land at Gatwick Airport instead of the much more distant Stansted Airport.

Under the current British-imposed restrictions, the number of Skytrain flights would be limited to one a day, with 189 seats each, during the offpeak season, and tickets could be sold no earlier than six hours before each flight. In Britain, only Laker would be able to sell tickets, in the U.S. they would be sold by Laker and a single approved sales agency, not by travel agents generally.

Laker's fare of $135 to London and 59 punds (about $101.50) back would remain unchanged.

In contrast to the total number of 1,323 seats Laker could offer in each director during the winter months, the IATA carriers would offer a total of 2,900 seats in each direction. They would offer the $256 fare under two plans: a standby arrangement where seats would be assigned after 4 a.m. on the day of the flight, and a program under which a passenger picks the week in which he wants to fly and buys a ticket at least 21 days in advance. The airline would inform the passenger of the exact date and time of flight within 10 days of the week of the flight. The IATA carriers would be using Heathrow Airport, which is much closer to downtown London than Stansted.

In a related development, the Justice Department has questioned whether the cut-rate transatlantic fare proposals of the IATA airlines are even legal.

In a petition filed with the Civil Aeronautics Board, which must approve the fares, the department said they are likely to be predatory in effect and could destroy the highly competitive charter airlines, which up to rate fares to Europe.

In the petition, filed by the Anti-trust Division, the department asked the CAB to investigate the lawfulness of the fares before letting them go into effect.

Noting that the charter airlines have complained that the proposed fares are "below cost" and therefore unreasonably low and predatory. Justice agreed that the differentials between the proposed discount fares and other existing scheduled fares are "so disproportionate" - as much as $500 - that questions are raised about whether both sets possibly could be cost-justified.

If the lower fares are not justified, the IATA carriers would be able to offset losses with the higher fares they would be charging the regular scheduled passengers, a Justice official explained.

At the same time, the low fares may lessen demand for the services of the charter carriers, who have no higher scheduled fares with which to offset any losses, and who therefore may be forced out of business.

That would have the scheduled carriers free to eliminate the discounts and raise their fares without any fear of the re-emergence of competition because of the strict capacity and airline limitations imposed on air services between Britain and the U.S. as a result of the recently signed agreement, Justice argued.

"The department always supported the increased availability of low-cost transportation, and these fares undoubtedly could bring substantial benefits to international travelers in the short run, and might therefore appear to be in the public interest . . ." the petition told the CAB. "We have reluctantly concluded, however, that these fares are likely to be predatory, coincidentally or intentionally, with respect to charter services."