The United States and its flag carriers have stepped up planning on how to react if no agreement is reached with Britain on scheduled air service between the two countries by the time the current agreemtn expires Wednesday.

Although the U.S. carriers had prepared contingency plans months ago for a possible cessation of service, reports that talks between the two governments are stalled make chances of reaching agreement by the deadline appear increasingly bleak. "We don't see anything significant happening over there," one airline source said yesterday.

The lack of a formal agreement would not necessarily mean that scheduled air service between the two long-time allies would cease. Without an agreement, twh two governments could very well agree to extend the terms of the existing but expiring agreement for a specified period of time during which negotiations would continue, and many airline and U.S. officials appear to be betting on this option.

According to aviation sources, however, should scheduled service cease, the airlines are prepared and committed to guaranteeing each person holding a ticket to a British destination transportation of one kind or another there at the same fare.

Pan American World Airways, for instance, has contingency plans to take its ticketed passengers by air to Amsterdam and Brussels and then to Britain by alternate sources of transportation, possibly including the use of charter flights and scheduled European service. Trans World Airlines has plans to operate similarly through Dublin and Paris.

The British carriers have contingency plans to take travelers to Canada.

In the past few days, Transportation Secretary Brock Adams met with the presidents of Pan Am, TWA, and Seabord World Airways, a cargo carrir, to bring himself up to date on their contingency plans, and he has been described as worried about the progress of the talks.

The negotiations, which began last September, are aimed at developing an agreement to replace the 31-year-old Bermuda Agreement which the British denounced a year ago. The British have complained that their carriers are not earning enough money on the transatlantic flights - the U.S. carriers have almost two-thirds of the traffice - and want an arrangement guarranteeing them exactly half.

Ambassador Alan S. Boyd, who leads that U.S. delegation, has said the U.S. is willing to try to meet what is considered the legitimate concerns of the British about excessive capacity across the Atlantic at times, but would not agree to a 50-50 split nor a government veto on the number of seats to be offered by the airlines.