Lithuania, one of 10 countries to join the EU in May, has become the first of the union's 25 member states to approve the new EU constitution.
Lithuania is the first to ratify the treaty since it was signed in Rome
Its parliament ratified the text, signed by EU leaders on 20 October, in a vote on Thursday.
The other 24 countries must all approve the document by parliamentary vote or referendum before it can take effect.
A No vote from any of the member states over the next two years would stop the constitution in its tracks.
The Lithuanian parliament approved the treaty by 84 votes to four, with three abstentions.
The opposition and some civic groups said the vote was purely political and was approved without any significant national debate, reports Steven Paulikus in Vilnius.
Thursday was the final day of the parliament's term, raising suspicion that current MPs wanted to take credit for the ratification before leaving office, he said.
Former French President Giscard d'Estaing, who oversaw the drafting of the constitution, sent a message of congratulations to Lithuania.
"This is a brave and a bold step... Thank you, men and women of Lithuania," he said in a letter read out in parliament.
European Commission spokesman Reijo Kemppinen said: "We congratulate them wholeheartedly for that. It is a very positive development indeed."
The first of the public votes is expected to take place in Spain in February next year.
There is intense scepticism in the UK, which could hold a referendum in 2006, while the outcome of the vote in France, Denmark and the Netherlands is also uncertain.
The constitution intends to make the union function more smoothly and includes a big expansion in the number of policy areas where countries will lose their national veto. It will also create a foreign minister's post.
But the EU's executive body, the European Commission, remains in a state of flux since its president, Jose Manuel Barroso, was forced to withdraw his proposed line-up of commissioners.
Some MEPs had threatened to reject his team of commissioners over the inclusion of Italian Rocco Buttiglione because of his conservative views on gays and the role of women.
Mr Buttiglione withdrew and Italian Foreign Minister Franco Frattini is now part of the revised team to be presented to the EU Parliament next week.