Portugal's Air Force confirmed that one of its surveillance aircraft had "inadvertently entered Finland's airspace" on Monday evening, according to the Portugal News, the country's biggest English-language newspaper.
Finland's defence ministry reported that the incursion took place above the Upinniemi peninsula in Kirkkonummi, on Finland's southern coast, at around 6.00pm on Monday.
Portugal's Air Force said the maritime aircraft, a Lockheed P-3C CUP+, had been on a Nato-led Assurance Measures operation over the Baltic Sea between Finland and Estonia, according to the paper.
The aircraft had carried out a "route correction and failed to prevent the radius of return exceeding about 0.3 nautical miles (500m) the limit of the air space," according to the Portugal News.
According to news service APTN, the aircraft had flown to the area from Poland.
Unlike Poland and Estonia, Finland is not a member of Nato, but does cooperate in peace-support operations and has participated with the military alliance in some of its exercises.