Defence Minister Antti Hakkanen said the reform, which enters into force on January 1 after being signed into law by the president, will result in an increase of 125,000 conscripts over five years.

"The number of Finnish reservists will be around one million in 2031," Hakkanen said in a statement.

Finland's reserve currently comprises around 900,000 citizens, and the eastern NATO country has a wartime strength of 280,000 soldiers.

"This and our other measures to bolster our defence signal that Finland ensures its security now and in the future," Hakkanen said.

Military service is mandatory for all Finnish men when they turn 18 and voluntary for women in the Nordic country of 5.6 million.

Conscripts may serve between six, nine or 12 months depending on their training.

The new age limit will apply to those liable for military service when the law comes into force.

Under the new rules, the availability of conscripts will be extended by 15 years for enlisted personnel and by 5 years for non-commissioned officers and officers.

Finland shares a 1,340-kilometre (830-mile) border with Russia and ended decades of military non-alignment by joining NATO in April 2023, just over a year after Russia's invasion of Ukraine.

Helsinki closed its eastern border with Russia in December 2023, suspecting Moscow of orchestrating the arrival of migrants to destabilise the country.