By Oana Lungescu
BBC correspondent in Brussels
Nato has confirmed it will formally admit seven more countries, all from the former communist bloc, on 2 April.
Nato's eastward expansion continues to irritate Russia
The nations' flags will be raised next to those of the 19 existing members at a foreign ministers' meeting in Brussels, a spokesman told the BBC.
They are Bulgaria, Estonia, Lithuania, Latvia, Romania, Slovakia and Slovenia.
This is the biggest expansion in the history of Nato, which was created in 1949 to defend Western Europe against the Soviet Union.
Nato will admit the seven new members one month before the European Union includes 10 more countries.
Over a decade since the collapse of the Berlin Wall, most of the former communist nations will become members of the two big Western clubs this spring - except Romania and Bulgaria, which won't join the EU for another three years.
The official ceremony will take place at Nato headquarters in just over a month's time, attended by all 26 foreign ministers.
The ministers will then prepare the ground for a Nato summit at Istanbul in June, where alliance leaders will try to boost Nato's role in Afghanistan and prepare to hand over to the EU a long-standing mission in Bosnia.
It is a sign of how much both organisations have changed since the end of the Cold War and the beginning of the war on terror.
Nato's eastwards expansion has been spearheaded by the United States, which received strong support from the former communist nations in the wake of the 11 September attacks.
The new Nato members provided air bases and troops for operations in Afghanistan and Iraq and are currently being considered for new US forward bases.
But Nato expansion continues to irritate Russia, especially since it includes the three Baltic countries, formerly part of the Soviet Union.
This week Moscow accused Nato of spying on its military facilities by deploying Awacs reconnaissance jets over future members Latvia and Lithuania.