Anders Fogh Rasmussen, who headed Nato from 2009-2014, is imploring Finland to join the military alliance now while Russia is focused on Ukraine.
Finnish and Swedish bids to join Nato would also send a powerful message to Vladimir Putin, according to Fogh Rasmussen in an interview with Radio Free Europe.
"Nato is prepared to welcome both Finland and Sweden. I would say an application from Finland and Sweden could be approved more or less overnight."
While Finnish Prime Minister Sanna Marin (SDP) has said any Nato membership decision would need to be weighed carefully, Fogh Rasmussen called on Finland to take swift action.
"I think it would be in the self-interest of Finland and Sweden to join Nato right now. Because now they have a window of opportunity. Putin is engaged [somewhere else], so that window might soon close again," he told the United States government-funded organisation.
Russia recently threatened "military and political consequences" against Finland and Sweden if they attempted to join Nato.
On that front, Fogh Rasmussen said Finnish and Swedish membership in the alliance was important to send a "clear signal that Nato's door remains open and Nato will never accept Putin as the doorman."