Finnair flights to London are among those being affected by strike action.
A 24-hour walk out by cabin crew will run until this afternoon (Monday).
Dozens of flights across the airline’s network have been cancelled.
The airline warned: “The strike will have considerable impact on Finnair operations and unfortunately, we are forced to cancel several flights.
“We apologise for the worry and inconvenience this may cause.”
Finnair said it would do its best to re-route passengers otherwise they could apply for a refund.
The dispute centres on plans to outsource certain cabin crew roles on long-haul routes.