The federal appeals court in New York City said yesterday that British Airways and Air France could begin flying the Concorde supersonic jet transport into John F. Kennedy International Airport there "forthwith."

The ruling set the stage for an immediate appeal by the Port Authority of New York and New Jersey to Supreme Court Justice Thurgood Marshall. The Port Authority operates Kennedy airport, and has resisted taking a final stand on the Concorde question for more than 16 months.

As a practical matter, Concorde operations could not begin at Kennedy for several weeks. Spokesmen for both European airlines expressed delight with the court's ruling, but said they were not ready to begin.

Both airlines would need "proving flights" without passengers into Kennedy before beginning regular service. Industry sources said it would probably take six weeks to begin passenger service, barring Supreme Court intervention.

The Concorde is already making 14 round trips a week to Dulles International Airport here. The Carter administration recently proposed regulations under which Concorde could come into other major U.S. cities on a local-option basis.

The administration also said that Concorde should in any case be given a 16-month test at New York, as was first proposed by the Ford administration.

The Second U.S. Circuit Court of Appeals, in acting yesterday, denied a petition by the Port Authority for a 30-day delay so it could adopt a local noise rule at Kennedy that would apply to the Concorde. Then the court approved the request from the two airlines to begin operations forthwith.

Concorde opponents, who have battled flights to Kennedy on the grounds that they will be too noisy, promised to block roads to the airport if the plane comes in.