The route was originally slated for launch in June 2020, but was put on hold due to the pandemic. BA will fly five times per week between London and Portland on a Boeing 787.
The route is among several new services that have either recently begun or are planned by the transatlantic joint venture partnership of American, British Airways, Finnair, Iberia, Aer Lingus and Level.
This month, Aer Lingus has launched flights from its new base in Manchester, England, to Orlando and New York JFK.
Finnair is planning launches next year of Helsinki-Dallas and Helsinki-Seattle. Iberia will fly Madrid-Dallas. And Level will fly Barcelona-Los Angeles beginning in March.