The Big Question: With Romania and Bulgaria joining the EU, how much bigger can it get?

Why are we asking the question now?

Romania and Bulgaria join the EU on 1 January, boosting the size of the bloc from 25 to 27 nations with a population of 492.8 million people, and the door is fast closing on would-be new members. One more country - Croatia - may sneak in under the wire to join the club, though not before the EU has revised its internal rulebook. (That makes 2010 a realistic target date for the Croats).

Any further expansion is much less certain. For one thing France, where voters feel threatened by enlargement, has promised its citizens a referendum on allowing in new members and is posing the real risk of a veto. For another, Europe's politicians are growing more sceptical about the EU's capacity to expand and absorb new members. Voters are far from convinced about the benefits of admitting Balkan countries. And Turkey's application has stirred opposition, particularly in France, Germany and Austria.

What do the voters think about the historic reunification of Europe?

They do not seem impressed. Britons used to be very keen on expansion of the EU. But in the latest Eurobarometer opinion poll, more than half - 51 per cent - came out against the EU's eastwards expansion. The number of opponents has jumped by nine points over the past six months with only 36 per cent supporting enlargement and 13 per cent undecided. A big divide has opened up between "old" and "new" Europe on the merits of expanding the club: support runs at 72 per cent in the 10 countries that joined the EU in 2004, but has dropped to an average 41 per cent in the other 15.

Are Romania and Bulgaria ready for membership?

Not really. Supporters of the two countries say that they are as prepared as Poland was in 2004, or Greece in 1981. But only a couple of months ago the European Commission was coruscating in its criticism of Bulgaria's performance. Since both Romania and Bulgaria had been promised entry, the only possible sanction would have been a one-year delay. But given that membership was guaranteed in 2008 anyway that might have been counter-productive, removing any incentive to keep on with reforms in 2007.

EU leaders realised that they had painted themselves into a corner and, earlier this month, decided to change the rules. From now on, no wannabe country will be given a date for membership. Instead, nations will only be given the green light to join when they are judged able to do so. Fears about Bulgaria's lack of readiness were underlined last week when the European Commission said it was so concerned about the standards of maintenance in Bulgarian aviation that it was placing special restrictions on the country's airlines.

Are fears of more immigration and crime justified?

When the EU admitted 10, mainly ex-Communist, nations in 2004 many tens of thousands more Poles than predicted arrived in the UK. This time, the Government has imposed restrictions on Romanians and Bulgarians who lack particular skills - though whether these will work in practice is another matter.

More worrying is Bulgaria's failure to get corruption and organised crime under control. The Commission's last progress report said that the country has managed "few concrete examples of investigations or prosecution of charges of high-level corruption", that there have been "no successful prosecutions for money laundering" and the number of cases related to organised crime were low.

Contract killings of persons rarely result in successful investigations and prosecutions," it said, adding: "Trafficking of new-born babies, involving pregnant women giving birth abroad, has not been halted." It should be said, however, that most of these problems are likely to remain in Bulgaria rather than coming to Britain.

How will the EU cope with having yet more MEPs and European Commissioners?

Rules are already in place on how many Euro-MPs the two countries can send and how many votes their ministers will have on new laws. Romanian and Bulgarian membership means two more official languages to work in - though, by coincidence, Irish is to gain full status in January too, increasing the strain on the interpreters.

There will now be 27 European Commissioners rather than the current 25, which can only make meetings even longer. The Brussels machine has already proved that it can manage with these numbers since, for a brief period in 2004 there were 30 men and women sitting around the Commission table. Nevertheless, it hardly makes for speedy decision-making.

Does the EU need new structures?

Most observers agree that the EU's Heath-Robinson machinery needs an overhaul. After all, this is basically the same structure that was used by the original six member states, one that has had to adapt to successive enlargements. The current situation imposes restrictions. For example, the existing treaties cap the number of European Commissioners at 27. Some solution must be found before the EU can admit Croatia to the club.

The trouble is that changes finally agreed by the EU's member states were rejected by the voters in referendums in France and the Netherlands. Now the issue of what will replace the constitution is returning to the fore. The most plausible answer is one that extracts key elements from the defunct constitution - such as a change to the voting system and the creation of a permanent president of the Council, where EU member states meet. These could then be put in a mini-treaty. Optimists hope for an agreement among all 27 EU nations by the end of next year though that seems over-ambitious.

Why does the EU go on expanding when voters seem opposed to it?

It's sometimes easy to forget the main point about Europe's expansion: that it has been an outstanding success. The last enlargement boosted Europe's economic performance and created a trading bloc with a much bigger population than that of America. Moreover it anchored young democracies within the European family, just as Greece, Spain and Portugal were brought in by a previous generation.

The lure of EU membership has been crucial in persuading politicians in the Balkans to curb the sectarian conflicts of the past. The prospect of joining the European family is vital for reformers in ex-Soviet nations like Ukraine. Overall, the expansion of the EU has been a force for peace, prosperity and stability.

Should the European Union expand further?

Yes...

* Previous enlargements have been a big economic success, forging a huge single market

* Balkan countries and ex-Soviet nations need to be brought into Europe for the sake of regional stability

* Turkish membership would help bridge the divide with the Muslim world

No...

* The EU has lost its sense of direction with 25 countries, let alone 27

* Europe has geographical and cultural limits, and its borders do not extend to Turkey

* Voters in the current EU countries do not support the idea of enlarging Europe further