The British Prime Minister, Mr Tony Blair today hailed the expansion of the EU as a "great day for Europe and a great day for Britain".
Mr Blair said admitting 10 new countries was bound to cause uncertainty and concern. But previous experience showed that enlargement was good for both existing members and the newcomers, he said.
He described the move as a "great opportunity" for Britain because many of the new countries shared the UK's strategic vision for the EU, he said.
"It is certainly a great day for Europe because the reunification of Europe after all the torments of the 20th Century but it is also a great opportunity for Britain.
"Britain has a tremendous opportunity because Europe is enlarging and the countries coming into Europe share our view of Europe as a Europe of independent nation states working together for the common good of all."
Speaking at the Dublin summit to mark enlargement Mr Blair said ratifying the new EU constitution in a referendum was "vital" to Britain's interests.
"It is a very, very big decisions this, there is no doubt about it," he said in a round of TV interviews.
"We have got to decide as a country whether Britain takes its rightful place at the centre of European decision-making and influence in this new enlarged influence or whether it retreats to the margins.
"Now I hope very much we keep our place at the centre of Europe. I think it is vital for our country. But in the end that is a decision that people have got to take."
PA