U.S. and British negotiators neared a midnight deadline without reaching agreement on a new pact governing scheduled air service between the two countries.

However, "chances are extremely good" for a new agreement, the chief American negotiator at the London talks said during a break.

"While a few important issues remain, there is the strongest possibility the deadline will be met and air services will continue without disruption," Special Ambassador Alan S. Boyd said last night.

Without an agreement - at least in principle - signed by both sides before the old accord expired (at midnight June 21 Washington time) scheduled flights of American carriers would not be permitted to land in London or Hong kong and scheduled flights of British Airways would not be permitted to land in the United States. Charter flights would not be affected.

In preparation for a possible deadlock, airlines of both countries set up plans to move passengers to their destinations by flights on other carriers or on trains or ships where possible from nearly locations in Europe and Canada.

American and British negotiators have been meeting in both London and Washington since September in an effort to resolve their differences and produce a new bilateral agreement by today. One year ago yesterday Britain gave the United States notice that it wished to terminate the Bermuda Air Transport Agreement that had governed air transportation between the two countries since 1946. Under the provisions of the agreement, it expires one year after either country gives notice of termination.

In general terms, the British said they wanted a guarantee of half the passengers moving between the two countries for British carriers; U.S. carriers have been carrying between 55 and 60 per cent of the passengers. The Bermuda agreement was based on the principle that both countries should have an equal opportunity to compete in the market, but that the market should not arbitrarily be divided up ahead of time.

Transportation Secretary Brock Adams told a news conference yesterday that it appeared "more likely each hour" that scheduled air service between the two long-time allies would end at midnight. He said that Ambassador Alan S. Boyd, who heads the U.S. delegation in London, had reported from London during the afternoon that chances of a settlement before midnight appeared "questionable."

Adams said that Boyd, who had been in telephone contact with President Carter, asked to be allowed to keep the talks going until the last minute.

Before he left for London in May for the final six-week session, Boyd had predicted that an agreement could be elusive until the end. "You don't really know until you get to the wire whether you're going to have an agreement or you aren't," he said. then.

"We're two civilized countries, we've been allies for a long time, and we have a great community of interest, and we're going to have to reach an agreement."

While talk in Washington was pessimistic about the outcome of the negotiations, the talk in London was optimistic. Prime Minister James Callaghan told Parliament yesterday he expected an agreement to be completed before the deadline "We are getting closer to agreement and I hope they can complete the negotiations satisfactorily today," he said.

An unusually somber Adams yesterday laid blame "for his unfortunate actopm - if it occurs" on the British government. He said the United States had already agreed to restrictive provisions that violate basic U.S. philosophy to encourage competitiion "simply because we must be able to land in foreign airports."

The British have been insistent that the number of flights be cut in order to increase the percentage of full flights. Preliminary soundings from London indicate the United States may have agreed to British demands to limit the number of carriers flying routes, even giving some routes to one U.S. or British carrier without any competition, and keeping some U.S. flights from stopping in London and picking up passengers to go on to other European cities.

"I think we have tried to meet a lot of their legitimate concers, but there are certain points beyond which we cannot go," Adams said yesterday.