The signing ceremony at Luxembourg's Neumuenster Abbey on Monday opened the way for the EU's second wave of enlargement into eastern Europe a year after the bloc's "big bang" expansion brought 10, mostly ex-communist countries into the bloc.
"Today, we are celebrating the reunion between European history and geography," Luxembourg Prime Minister Jean-Claude Juncker, who holds the EU's rotating presidency, said.
Bulgaria and Romania will enter the EU on 1 January 2007, provided they carry out tough reforms to fight corruption, strengthen border controls, beef up judicial and administrative systems and improve rules on state aid to industry.
If they do not, their membership could be delayed until 2008, according to the 860-page treaty signed by Bulgarian and Romanian leaders and the EU foreign ministers.
EC president Barroso pledged EU
help for Romania and Bulgaria
European Commission President Jose Manuel Barroso told Bulgaria and Romania that his executive would give the countries all the help needed to avoid the delay.
"Rest assured that we will also be working with you to overcome any difficulties as you make your final push between now and January 2007," Barroso said.
EU entry will irrevocably lock Bulgaria and Romania into the bloc's zone of prosperity and provide billions of euros in aid to repair dilapidated roads, clean up the environment and modernise their often outmoded industries.
It should provide crucial foreign investment in the states whose economic output per capita is well below 40% of the EU average.
Unready to join?
The expansion will also extend the EU's position as the world's biggest trading bloc.
"Today, we are celebrating the reunion between European history and geography"
Luxembourg Prime Minister
Enlargement will add more than 30 million to the bloc's 454 million people and move its borders to the Black Sea.
Some Western diplomats believe the countries, especially Romania, are not yet ready the join the EU because of rampant corruption, inefficient courts and insufficient protection of the Roma ethnic minority.
Their admission could add to "enlargement fatigue" in Western Europe, which many analysts say is a factor in opposition to the EU constitution in the campaign for France's 29 May referendum.