The parliaments of each existing NATO member must approve Finland and Sweden's accession before it can happen.
Finland and Sweden are a step closer to joining the NATO military alliance.
Both countries, who decided to apply for membership following Russia's invasion of Ukraine, had their bids approved at a summit last week.
The next step, however, is to get their accession ratified by the parliaments of NATO member countries.
On Tuesday, that process officially began when NATO's 30 allies signed an accession protocol. It means Helsinki and Stockholm can participate in NATO meetings and have greater access to intelligence but will not be protected by the NATO defence clause -- that an attack on one ally is an attack against all -- until ratification. That is likely to take up to a year.
“This is truly a historic moment for Finland, for Sweden and for NATO,” said Secretary-General Jens Stoltenberg.
"With 32 nations around the table, we will be even stronger ... at a time when we are facing the most serious security crisis in decades".
The formal approval comes after NATO's 30 ambassadors and permanent representatives approved Finland and Sweden's accession at last week's summit.
It also allows Finnish and Swedish representatives to attend all NATO meetings, even if they do not yet have voting rights, or are protected by the alliance's defence clause.
The last hurdle for the two countries could be receiving parliamentary approval from Turkey. NATO accession must be formally approved by all 30 member states, which gives each a blocking right.
Last week, Turkish leader Recep Tayyip Erdogan warned that Ankara could still block the accession process if Finland and Sweden fail to fully meet Turkey’s demand.
Turkish President Recep Tayyip Erdogan warned that Ankara could block the process if the two countries failed to grant Turkey’s demands for the extradition of people it views as terror suspects. The people wanted in Turkey have links to outlawed Kurdish groups or the network of an exiled cleric accused of a failed 2016 coup in Turkey.
But Stoltenberg said on Tuesday he expected no change of heart.
“There were security concerns that needed to be addressed, and we did what we always do at NATO," he said. "We found common ground.”
Swedish Foreign Minister Ann Linde also told reporters that a specific list of people had not been part of their agreement with Turkey.
“We will honour that memorandum and follow up on that,” she said adding her government’s actions would always ”comply with the Swedish legislation ...[and] international law.”
Given all the different legislative procedures in NATO's 30 members, it could still take several more months before Finland and Sweden become official members.
Germany’s parliament has said it would ratify the membership bids on Friday, while Estonia is hoping to be the first.