Nato's 30 allies signed an accession protocol for Finland and Sweden on Tuesday, taking the two Nordic countries a step closer to membership.
It will allow them to join the nuclear-armed alliance once allied parliaments ratify the decision, the most significant expansion of Nato since the mid-1990s.
The move increases Russia’s isolation since its invasion of neighbouring Ukraine in February and the subsequent conflict.
The protocol means Helsinki and Stockholm can participate in Nato meetings and have greater access to intelligence but they 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.
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 the two countries to join the military pact.
"This is truly a historic moment," Nato Secretary General Jens Stoltenberg said alongside the foreign ministers of the two countries. "With 32 nations around the table, we will be even stronger."
Nato's 30 ambassadors and Mr Stoltenberg stood together for a photo in which the foreign ministers of Sweden and Finland held up their signed protocols, before breaking out into applause.
Swedish Foreign Minister Ann Linde said on Twitter: "Thank you for your support! Now the process of ratification by each of the allies begins. Look forward to working together in ensuring our collective security."
Despite the agreement in the alliance, parliamentary approval in member state Turkey could still pose problems for their final inclusion as members.
Turkish President Tayyip Erdogan issued a warning at last week's summit in Madrid that Finland and Sweden must first keep the promises made to Turkey in a deal or ratification will not be sent to the Turkish Parliament.
Historic Nato summit - in pictures
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US President Joe Biden speaks at a news conference on the final day of the Nato summit in Madrid. AP -
British Prime Minister Boris Johnson holds a news conference. PA -
Turkish President Recep Tayyip Erdogan meets German Chancellor Olaf Scholz. AFP -
French President Emmanuel Macron speaks during the final day of the Nato summit. AP -
Turkish President Recep Tayyip Erdogan and US President Joe Biden shake hands as Nato secretary general Jens Stoltenberg and British Prime Minister Boris Johnson look on at a Nato summit in Madrid. Reuters -
Heads of state of Nato member countries and their spouses pose for a group photo during a visit to the Prado Museum, in Madrid. AFP -
French president Emmanuel Macron, right, and Greek Prime Minister Kyriakos Mitsotakis during the Prado Museum visit. AP -
Mr Biden takes a selfie with Maltese Prime Minister Robert Abela and his wife Lydia Abela as they visit the Prado Museum. AP -
British Prime Minister Boris Johnson, the first lady of France, Brigitte Macron, right, and Belgian Prime Minister Alexander De Croo's wife Annik Penders have a conversation at the museum. AP -
Spanish police stand in front of protesters during an anti-Nato demonstration near Tirso de Molina square in Madrid. AFP -
Ukrainian President Volodymyr Zelenskyy appears on a giant screen as he delivers a statement at the start of the first plenary session of the Nato summit. AFP -
World leaders pose for a photo during the summit. Reuters -
US President Joe Biden, left, and Nato Secretary General Jens Stoltenberg. AP -
Maria Begona Gomez, wife of Spanish Prime Minister Pedro Sanchez, the first Lady of Lithuania, Diana Nausediene, first lady of Malta, Lydia Abela, and Gauthier Destenay, the husband of Luxembourg's Prime Minister, before a visit to the royal site of San Idelfonso in Segovia. EPA -
Finnish Foreign Minister Pekka Haavisto, Turkish Foreign Minister Mevlut Cavusoglu, Jens Stoltenberg, Turkish President Recep Tayyip Erdogan, Finland's President Sauli Niinisto, Sweden's Prime Minister Magdalena Andersson and Sweden's Foreign Minister Ann Linde pose for a picture after signing an agreement in Madrid. AP -
King Felipe VI of Spain addresses leaders during a dinner at the Royal Palace in Madrid. Reuters -
Mr Johnson meets Australian Prime Minister Anthony Albanese. PA -
Canada's Prime Minister Justin Trudeau and European Commission President Ursula von der Leyen attend a reception at the Royal Palace in Madrid. AP -
Spain's King Felipe VI and Queen Letizia greet German Chancellor Olaf Scholz, right, during a royal reception for heads of governments and states. EPA -
King Felipe VI, left, Mr Sanchez, second left and Mr Macron before the dinner. AP -
Nato leaders pose for a 'family photo' with King Felipe and Queen Letizia in Madrid. Reuters -
Denmark's Prime Minister Mette Frederiksen, left, with Mr Biden and Mr Macron. EPA -
Mr Biden, left, and King Felipe arrive for a meeting at the Royal Palace in Madrid. Reuters -
Mr Johnson talks to journalists on his plane during a flight from Germany – where he was attending the G7 summit – to the Nato summit in the Spanish capital. PA -
Mr Erdogan, second left, meets Mr Stoltenberg, Mr Niinisto of Finland and Ms Andersson of Sweden before the summit. Reuters -
Mr Biden shakes hands with Prime Minister Sanchez of Spain at the Palace of Moncloa, in Madrid. AFP -
Mr Erdogan, centre, arrives at the Torreon air base in Madrid. AP -
Mr Sanchez, right, speaks with Mr Stoltenberg before the summit. EPA -
US first lady Jill Biden, centre left, and Spain's Queen Letizia speak with a family during a visit to a reception centre for Ukrainian refugees in Madrid. AP -
Spanish police patrol outside the Ifema convention centre before the Nato summit in Madrid. EPA -
Mr Sanchez and his New Zealand counterpart Jacinda Ardern hold the jerseys of their national football teams during a meeting at Moncloa Palace. EPA -
Finland's President Niinisto speaks during a briefing in Madrid before the Nato summit. Reuters -
Australian Prime Minister Anthony Albanese speaks to the media after arriving at Torrejon Airbase in Madrid. EPA -
Spanish police officers control traffic on the Spain-France border in Irun. Reuters -
Mr Stoltenberg speaks at a press conference to preview the Nato summit at the alliance's headquarters in Brussels, Belgium. AFP -
Spanish police use sniffer dogs to check a podium at the Ifema centre in Madrid. EPA -
US first lady Jill Biden waves on arrival at Torrejon air base in Madrid. Reuters
After weeks of diplomacy, Mr Erdogan and his Finnish and Swedish counterparts agreed on a series of security measures to allow the two Nordic countries to overcome the Turkish veto that Ankara had imposed in May due to its concerns about terrorism.
But he insisted Ankara could still block the process if the two countries fail to fully meet Turkey’s demand to extradite suspected terrorists with links to outlawed Kurdish groups or the network of an exiled cleric accused of a failed 2016 coup in Turkey.
According to the signed memorandum, Finland and Sweden pledged not to support the Kurdish militant PKK and YPG groups or the network of US-based cleric Fethullah Gulen, which Ankara says staged a 2016 coup attempt and which it labels a terrorist organisation with the acronym Feto.
Mr Stoltenberg said he expected no change of heart. “There were security concerns that needed to be addressed," he said. "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 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 provide Nato with more clout, especially in the face of Moscow’s military threat.
Tuesday’s signing has brought both nations deeper into Nato’s fold already. As close partners, they already attended some meetings involving issues that immediately affected them. As they were officially invited, they can attend all meetings of the ambassadors even if they do not yet have any voting rights.
