Brussels - Five years after its biggest-ever enlargement, the European Union is still struggling to accept that it really is one union, not two separate blocs called "old" and "new."

Talk of "old Europe" and "new Europe" is "very much a common use: it's very easy and everybody knows who you mean by it, so I think it will stick with us for a while," Maros Sefcovic, Slovakia's EU ambassador since enlargement, told the German Press Agency dpa.

On May 1, 2004, the EU accepted Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia as members in the largest expansion in its history.

The move brought the EU some 73 million more citizens, boosted the number of member states from 15 to 25, gave it nine new official languages and shifted its border almost 1,000 kilometres East.

To start with, it slammed the brakes on the EU's ability to take decisions and pass laws, as both new and old member states tried to work out how to cut deals with a greatly increased number of partners.

"The dynamics are different: everyone had to learn them from the beginning, and it's a tough job because nobody was ready," said Piotr Kaczynski, an expert on EU constitutional affairs at the Centre for European Policy Studies (CEPS) in Brussels.

Analysts say that, within 18 months, the bloc's decision-makers were back up to speed.

"In terms of legislation, the rate has even increased over the last two years. The machinery functions and it's doing its job," Sara Hagemann, a leading expert on post-enlargement decision-making at the European Policy Centre (EPC) in Brussels, said.

But the sheer scale of the new EU - now at 27 members after Romania and Bulgaria joined in 2007 - means that it takes much more work, and much more negotiation, to reach those decisions.

"There is a bit of fatigue in the system: everyone is crying out for reform," Hagemann said.

Nowhere have those problems been more evident than in the EU's attempts to create a joint foreign policy.

Rows between Cyprus and Turkey, and between former-Communist members and Russia, have poisoned relations with some of the EU's key partners.

The EU's common foreign policy "is somehow suffering from enlargement ... It's led to a situation where the process to define a common strategy has become way more difficult," said Markus Kaim, head of International Security at the SWP - German Institute for International and Security Affairs in Berlin.

Similar question marks hang over the economic impact of the expansion. On February 20, the EU's executive, the European Commission, published a report stressing the economic benefits which the Big Bang had brought.

The report said that enlargement boosted growth in new and old member states, stimulated trade between them and gave labour- intensive industries such as construction a much-needed boost.

But it also encouraged banks in old members such as Austria to invest massively in ill-regulated states such as Latvia and Hungary.

And the financial meltdown which hit those two countries in the autumn left analysts warning that, far from boosting growth in the West, the new members could now be about to cripple it.

If the subsidiaries of Western European banks fail in the new member states, "it will almost certainly have an impact on the euro zone," warned Thomas Mirow, the head of the European Bank for Reconstruction and Development, on February 27.

All those issues have raised hackles in old EU member states. Populist politicians have blamed the enlargement for everything from mass migration and crime to financial meltdown and the rejection in France, the Netherlands and Ireland of new EU treaties.

Indeed, since the French and Dutch votes in early 2005, a new term - "enlargement fatigue" - has become common parlance in Brussels.

"In some old states, there is a relative lack of will to enlarge further: a predominant feeling that it would be best if the newer members would shut up and be governed and let us do business as before ," Kaczynski said.

But as the Big Bang enlargement approaches its fifth anniversary, experts say that the only way for the bloc to digest it and stop thinking in terms of "new" and "old" would be to enlarge still more.

"This will change the moment the next wave of new member states is taken into the EU: then they would become the new and we would become the old. But how long this will take is very difficult to predict," Sefcovic said.