Finnish air traffic controllers walked off the job on Monday to protest a reorganisation undertaken by airport authorities, their main union announced, but only minimal flight disturbances were expected.
The SLJY union, representing more than 90 percent of air traffic controllers, called on its members to walk off the job as of 1400 GMT and only return at 0500 GMT on Tuesday.
Controllers are protesting against the transfer of the Area Control Centre from Tampere -- located 160 kilometers (100 miles) north of Helsinki -- to the Helsinki airport.
The centre coordinates all flights in Finnish airspace and air traffic between different airports, with the exception of the zones immediately surrounding airports, handling about 270,000 flights a year.
"The transfer presents a problem from a security point of view," union boss Sami Fabritius said.
According to the union, once the centre is transferred to the Helsinki airport, any technical breakdown would have consequences for air traffic control in the entire country.
SLJY also fears jobs will be cut, seeing the reorganisation as a cost-cutting move.
Aviation authority Finavia said it expected some domestic flight cancellations but international flights would go ahead as scheduled.
"Flights will be handled by air traffic control supervisors" who are not unionised, Finavia said.
SLJY said however some international flights could experience delays.