Political instability in ex-communist countries joining the European Union on May 1 may undermine the bloc's already half-hearted drive at economic reform and closer co-operation, diplomats and analysts say.
Weak governments, fluid party systems and a surge of populist forces could make some of the 10 EU newcomers unpredictable partners for the current 15 members, hampering or delaying major policy decisions.
The arrival of new members, some keen to prove they will not be bullied by the bloc's heavyweights, may exacerbate existing divisions in the EU on issues such as attitudes towards the United States, economic policy and reform of Brussels' budget.
Three weeks before the EU's biggest ever expansion, Poland's left-wing government is about to resign with the succession unclear, Lithuania has impeached its president and Slovakia looks set to elect a controversial nationalist as president.
In the Czech Republic, where the president is euro-sceptic Vaclav Klaus, the government has lost its majority in parliament, while in Slovenia five cabinet ministers have resigned after a split in the ruling coalition.
Latvia has just appointed its 12th government in 13 years and neighbouring Estonia suffered a cabinet crisis last year.
"The EU could face a rough ride in the first years after enlargement," said Heather Grabbe, enlargement specialist at the Centre for European Reform in London.
But she said that despite many doubts, EU enlargement had been agreed precisely with the aim of stabilising central Europe's often fragile democracies 13 years after the fall of communism by locking the region in the EU's fold of prosperity.
Some EU diplomats say their biggest worry is Poland, by far the largest newcomer, where the pro-European government's imminent collapse could lead to an early general election and a victory for the anti-EU, populist self-defence party.
"Sometimes it's a combination of extreme nationalism and extreme Catholicism that is determining the political climate and the political debate in Poland," EU Enlargement Commissioner Guenter Verheugen said in a recent Reuters interview.
Even if populists do not win power, analysts warn that weak governments in Poland and elsewhere may not back tough decisions EU leaders need to make to revive the bloc's stagnant economy, overhaul its costly farm policy and streamline bureaucracy.
"A government with a weak popular mandate, like Poland's, will fear responsibility for decisions. It is likely to try to delay them and resort more often to national referenda," said former Polish foreign minister Dariusz Rosati. Poland may have softened its opposition to a proposed EU constitution, but President Aleksander Kwasniewski has said the charter will need referendum approval, which looks uncertain.
This attitude augurs ill for decisions facing the EU in the next year or two - the approval of its next long-term budget, co-ordination of foreign, defence and internal security policies as well as further harmonisation of financial regulation.
Diplomats say the newcomers will not form a unified group in the EU, instead joining existing coalitions in the bloc, which could at times deepen current divisions among member states.
"Co-operation between incoming EU members is volatile. They prefer to team up with current more experienced members," said Pawel Swieboda, EU director at the Polish Foreign Ministry. For example, small countries - new and old - are expected to join forces to defend a bigger role for the European Commission, the EU's executive, to protect their rights against the interests of the heavyweight member states.
Slovakia and other newcomers that have recently cut corporate tax rates to boost growth will join Britain and Ireland in opposing harmonisation of tax systems in the EU.
Small newcomers are likely to back demands for a radical shift in EU expenditure from farm subsidies to pro-growth projects, but Poland may be tempted to team up with France to block reform, given both countries' large agriculture sectors.
The mostly poor new members will strengthen the club of Mediterranean states battling demands from rich EU states such as Germany and Britain to slash regional development aid.
They will also boost the camp, led by Britain, of countries that advocate closer ties with the United States against France and Germany, which favour a more independent EU foreign policy and opposed the war on Iraq.