The 30 NATO allies have signed off on the accession protocols for Sweden and Finland, sending the membership bids of the two nations to the alliance capitals for legislative approvals.
The move further increases Russia's strategic isolation in the wake of its invasion of neighbouring Ukraine in February.
Alliance Secretary General Jens Stoltenberg said: "This is truly a historic moment for Finland, for Sweden and for NATO."
The 30 ambassadors and permanent representatives formally approved the decisions of last week's NATO summit when the alliance made the historic decision to invite Russia's neighbour Finland and Scandinavian partner Sweden to join the military club.
Despite the agreement in the alliance, parliamentary approval in member state Turkey could still pose problems for their final inclusion as members.
Last week, Turkish leader Recep Tayyip Erdogan warned that Ankara could still block the process if the two countries fail to fully meet Turkey's demand to extradite terror suspects with links to outlawed Kurdish groups or the network of an exiled cleric accused of a failed 2016 coup in Turkey.
He said Turkey's parliament could refuse to ratify the deal. This is a potent threat since NATO accession must be formally approved by all 30 member states, which gives each a blocking right.
Watch: Sweden's Foreign Minister Anne Linde says her country will contribute to NATO's collective defence.
Mr Stoltenberg said he expected no change of heart.
"There were security concerns that needed to be addressed. And we did what we always do at NATO. We found common ground."
Every alliance nation has different legislative challenges and procedures to deal with, and it could take several more months for the two countries to become official members.
"I look forward to a swift ratification process," said Finnish foreign minister Pekka Haavisto.
The Russian invasion of Ukraine has given the process added urgency. It will ensconce the two nations in the Western military alliance and give NATO more clout, especially in the face of Moscow's military threat.
"We will be even stronger and our people will be even safer as we face the biggest security crisis in decades," said Mr Stoltenberg.
Tuesday's signing-off does bring both nations deeper into NATO's fold already.
As close partners, they already attended some meetings that involved issues that immediately affected them.
As official invitees, they can attend all meetings of the ambassadors even if they do not yet have any voting rights.