Civil Aeronautics Board officials are trying to figure out how to deal with an apparently illegal attempt to influence the board's decision in awarding a new route to London.

Board officials yesterday placed on the public record a handwritten note sent by Western Airlines President Dominic P. Renda to CAB Chairman Marvin S. Cohen warning that the airline will lose $450,000 if it doesn't get the new route between Minneapolis and London.

The CAB is in the process of deciding between Western and Northern airlines.

Under the Bermuda II agreement governing air services between the United States and the United Kingdom, the United States is entitled to name a new city to receive nonstop services by on U.S. airline to London beginning this summer. The CAB last month tentatively decided that Minneapolis should get the new service but put off its decision on which carrier would be awarded the new service.

Because Northwest told the board during the proceeding that it should get the route because Western wouldn't have the airplanes to start the service on time, Western's Renda was apparently trying to make sure that Cohen and the CAB knew Western had made arrangements to get planes.

While in London attending counsultations with the British, Cohen found a copy of a Western press release and the handwritten note from Renda slipped under his hotel room door, according to CAB sources.

The press release announced that Western had agreed to buy three long-range McDornell Douglas DC10 aircrafts from Pan American World Airways.