Sweden will apply for membership in Nato as a deterrent against Russian aggression, Swedish Prime Minister Magdalena Andersson said on Monday, reversing two centuries of military non-alignment.

"The government has decided to inform Nato that Sweden wants to become a member of the alliance," Andersson told reporters a day after neighbouring Finland made a similar announcement.

"We are leaving one era and beginning another," she said, adding that Sweden's Nato ambassador would "shortly" inform Nato.

Sweden and Finland have both expressed a desire to act in lockstep on Nato membership and submit their applications jointly.

"We expect it shouldn't take more than a year" for the alliance's 30 members to unanimously ratify Sweden's membership application, Andersson said.

The announcement was expected after her Social Democratic party on Sunday backed membership, in a dramatic turnaround after having opposed the idea since the birth of the Western military alliance.

A broad majority in Sweden's parliament is in favour of membership. (AFP)