EU leaders have signed a treaty in the Portuguese capital, Lisbon, that is expected to greatly alter the way the 27-nation body operates.
The treaty was signed at Lisbon's historic Jeronimos monastery
The treaty creates an EU president and a more powerful foreign policy chief.
The document, signed at a ceremony at the city's historic Jeronimos Monastery, also scraps veto powers in many policy areas.
It is a replacement for the EU constitution, which was abandoned following French and Dutch opposition.
EU leaders insist that the two texts are in no way equivalent.
But the Lisbon treaty incorporates some of the draft constitution's key reforms, and several governments face domestic pressure over the document.
In a speech before the signing, European Commission President Jose Manuel Barroso called on European leaders to use the treaty to make freedom, prosperity and solidarity an everyday reality for all European citizens.
KEY LISBON TREATY REFORMS
Creates new European Council president
New foreign policy supremo to increase EU profile
Commissioners reduced from 27 to 18
Removes national vetoes in around 50 policy areas
Voting weights between member states redistributed
No reference to EU symbols such as the flag and anthem
Treaty faces referendum in Ireland and must be ratified by all other EU parliaments
"From this old continent, a new Europe is born," he said.
Portuguese Prime Minister Jose Socrates, whose country holds the rotating EU presidency, said the treaty would create a more modern, efficient and democratic union.
"The world needs a stronger Europe," he said.
The leaders signed the treaty, translated into the EU's 23 official languages, using specially engraved silver fountain pens as a choir sang Beethoven's Ode to Joy.
UK Prime Minister Gordon Brown signed the treaty later in the day after missing the ceremony, citing a prior engagement in the British parliament.
Foreign Secretary David Miliband attended the signing ceremony.
Mr Brown signed the treaty hours after fellow EU leaders
The UK's opposition Conservatives accused Mr Brown of "not having the guts" to sign the treaty, which is politically controversial in Britain, in public.
Having started this year with a celebration of its 50th birthday, the EU hopes the signing of the Lisbon treaty will end the serious mid-life crisis brought about by the death of the constitution, the BBC's Oana Lungescu reports.
There will be a lot of relief, said a senior European diplomat, but also some apprehension about what happens next.
Ireland is the only country planning to hold a referendum, but most voters there seem either undecided or indifferent.
Parliaments in Britain, the Netherlands and Denmark are also expected to give a turbulent reception to the 250-page text.
However, Germany, France and Poland have pledged to be among the first to ratify it, so that the new reforms can come into force in 2009 as planned.
The treaty is a slimmed-down version of the European constitution, with a more modest name and without any reference to EU symbols such as the flag and anthem.
It is meant to ease decision-making, by scrapping national vetoes in some 50 policy areas, including sensitive ones such as police and judicial co-operation.
There will also be a foreign policy chief, controlling a big budget and thousands of diplomats and officials, and a permanent EU president appointed for up to five years.
But some already fear that instead of giving Europe a strong single voice in the world, the new posts will only generate more rivalry, our correspondent adds.