And then there were 25

FIRST there were six. It was 1957, just 12 years after the end of the Second World War, when six countries - France, Germany, Belgium, Italy, the Netherlands and Luxembourg - signed the Treaty of Rome to form the European Economic Community. The idea was twofold: to create a larger market for the benefit of the member countries, and to heal some of the divisions wrought by the war.

Nearly 50 years on, and that six has swollen to 25. The Common Market has undergone many changes in those five decades, not least of which was adding political unity to its original aims, but the ambitions of its latest expansion bear a striking similarity to that of its foundation in at least one key respect.

Of the ten countries joining what is now the European Union, making a market of 450 million people, eight are from the former Eastern bloc, within the Soviet Union's sphere of influence. More than a decade after the end of the Cold War, these emerging democracies are now judged ready to take their seats at the world's largest economic federation.

In a continuation of the principles of the Treaty of Rome, tomorrow's enlargement is seen by its supporters as an historic opportunity to unify Europe after centuries of bitter division and conflict, of which the Cold War was only a brief, albeit key, episode. The Iron Curtain which once rent the continent in two has gone; in its place a welcome mat.

But what will actually happen tomorrow, and what will enlargement mean for the existing EU countries?

Q What does enlargement involve?

A Ten countries are joining the EU - Estonia, Latvia, Lithuania, Poland, the Czech Republic, Slovakia, Slovenia, Hungary, Cyprus and Malta - bringing the number of EU members to 25. This is the fourth expansion in its history: the UK, Ireland and Denmark joined the original six in 1973; Greece in 1981; Spain and Portugal in 1986, and Austria, Finland and Sweden in 1995, when the EEC became the EU.

Q Why do they want to join?

A The principle benefit is seen to be economic. Although the former Eastern bloc countries have made great strides since the collapse of communism in Eastern Europe, their economies are still some way behind their Western cousins. The average GDP per head of the new member states is 40 per cent of the EU average, although it ranges from 72 per cent in Malta to 35 per cent in Latvia.

Joining the EU gives them access to the largest free market in the world, now comprising some 450 million people, and so an opportunity to both buy and sell goods to the rest of the EU free from import/export duties.

All the new members apart from Cyprus held referendums last year, with an average Yes vote of 78 per cent, ranging from 54 per cent in Malta to 92 per cent in Slovakia. Cyprus held a referendum earlier this week on whether the Turkish half should accompany them into the EU, but this was rejected by the Greek south, to the annoyance of EU negotiators who had seen entry as a way of reunifying the island.

Q Why does the EU want to expand?

A Although all the new members are poorer than the existing ones, enlargement is still seen as a way of promoting economic growth through removing trade barriers. In order to become members, the new countries have also had to carry out economic and social reforms, making them more attractive to inward investment.

The political dimension is that the new members, particularly those from Eastern Europe, will become "locked-in" to the European project, increasing stability and going some way to ensuring the divisions of the past are not re-opened. A larger EU, in turn, makes it a more influential body.

Q What did the new countries have to do to join?

A They had to demonstrate that they were genuine democracies, a particular requirement for the former Eastern bloc countries, and also that the rule of law and human rights were guaranteed. In addition, their economies had to be considered capable of bearing the strain of being in a competitive market of 450 million consumers.

Q How much will this expansion cost?

A The EU has budgeted to spend 40 billion euros on the new member states over the next three years, although they will pay 15 billion euros into the EU budget, making it a net outlay of 25 billion euros.

Q How will the EU change?

A This is one of the major unanswered questions. Many people believe it will be harder for France and Germany to determine the direction of the EU, as they have done in the past. This could either lead them to enlist the UK's support on more issues, or to the creation of a new dominant bloc. Many of the Eastern European countries are also more pro-US than existing members, meaning there could be a shift in the EU's outlook, although maybe only in the long-term.

On a more day-to-day level, the fear is that with 25 countries, the EU will become too unwieldy and unable to make major decisions. It was with this in mind that the European Convention, headed by former French president Valery Giscard D'Estaing, drew up the European constitution, designed to streamline the EU's institutions. This has yet to be ratified, and Tony Blair has promised there will be a referendum in Britain on the constitution next year.

As far as the institutions go at present, the European Commission will go from 20 to 25 members, with five countries, including Britain, losing their second commissioner (posts filled by the UK at present by Neil Kinnock and Chris Patten). The European Parliament will go from 626 seats to 732.

Q Will it mean economic migrants will come to the West looking for jobs?

A Some will, although forecasts of how many vary widely, with one EU study suggesting it will be 220,000 a year spread across the 15 existing members.

Part of the single market is that citizens of one EU country can live and work in any member country, but this has led to fears that large numbers of people will leave the new, poorer, member states, tempted by the higher wages and benefits on offer in other countries. As a result, a provision of enlargement is that existing states are allowed to limit some of these rights for seven years, an opportunity most of them have taken.

The UK Government originally decreed that citizens of the new EU countries will be treated the same as those from any other EU member, but growing public disquiet forced a U-turn. Under the new plans, anyone from a new member state wanting to work in Britain will first have to get a work registration certificate from their employer, making it easier for the Government to monitor how many are coming here. Also, migrants will not be able to claim benefits for at least two years, and those who cannot support themselves will be thrown out of the country.

Seven years is considered to be how long it will take for the economies of the new member countries to catch up with the rest of the EU, making economic migration unappealing.

Q Will these countries join the euro?

A All of the new members have committed themselves to joining the single currency, but only when it is appropriate for their national economies. Of the 12 existing members, only the UK and Denmark, which have negotiated opt-out clauses, and Sweden, which voted against in a referendum, are not in the eurozone.

Q Is this the end for EU expansion?

A No. Three more countries are candidates to join - Bulgaria, Romania and Turkey. Bulgaria and Romania are expected to become members in 2007, although talks on Turkey's accession will not begin until next year, with its record on human rights a possible sticking point.

Croatia and Macedonia have also officially applied to join the EU, and European countries in the former Soviet Union, such as the Ukraine, may also consider applying to join. Norway and Switzerland have no plans to add their names to the list, although both have come close in the past.