"We are facing adversaries who target our common values in cyberspace: freedom, truth, trust," centre director Merle Maigre said at the ceremony.

"To build resilience we need to come together. That is why I am glad to welcome Portugal as together we are stronger," she added.

The centre was founded in 2008 in the capital of cyber-savvy Estonia, ranked as having one of the world's highest internet user rates, which itself had come under attack the previous year.

Estonia accused Russia, NATO's old Cold War foe, of being behind the attacks on its official sites and information networks.

At the centre, data experts from across Europe and the United States work to protect the information networks of the Western defence alliance's 29 countries.

The centre's current members are Austria, Belgium, the Czech Republic, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, the Netherlands, Poland, Portugal, Slovakia, Spain, Sweden, Turkey, the United Kingdom and the United States.

Australia, Norway and Japan have said they also plan to join.