Never one to take lightly a coup by his chief rival, American Airlines Chairman Robert L. Crandall yesterday offered to top a $400 million bid by United Airlines to acquire Pan Am's coveted London routes.

But the surprise announcement appears unlikely to impede United's efforts to acquire the prized access to London's Heathrow Airport.

Pan American World Airways Chairman Thomas G. Plaskett indicated yesterday he isn't interested in American's offer, referring to the deal with United as a "legally binding agreement."

"Maybe there is some misunderstanding regarding the scope of our agreement with United," Plaskett said. "Among other things, it envisions a long-term marketing program and other cooperative arrangements," he said.

The deal, announced Tuesday, links United and Pan Am's frequent-flier programs and calls for the two carriers to coordinate their flights at international airports in Los Angeles and Miami to boost feeder traffic to Pan Am's other destinations.

Crandall said he had talked to Plaskett as recently as the day before the announcement of the United deal about acquiring the transatlantic routes. Plaskett said that American had expressed interest only in the routes, not in a broader agreement.

Airline industry analysts said yesterday that the United deal appeared to hold more promise for Pan Am in the long-term because Pan Am and American compete against each other in more markets than do Pan Am and United.