The Liberal Democrats will next week signal a radical hardening of their stance on Europe with a new scepticism about enlarging the European Union.
In a speech to the party's conference in Bournemouth, Chris Davies, leader of the party in the European Parliament, will argue for a block on further entry to the EU until constitutional reforms are in place.
Mr Davies will announce that Liberal Democrat MEPs may even vote against admitting countries such as Bulgaria, Turkey and Romania, which have applied to join. He will warn: "You can't cross the threshold till we've made sure the structure is safe."
Mr Davies will warn that member states could be "skating on thin ice" and will sharply criticise the way in which the European Union operates. He will argue that further enlargement should be delayed until the EU is strengthened and the new constitution adopted.
"We must make clear that there is no inevitability to accession negotiations commencing or to them being concluded," he will say.
His speech, approved by the party's MEPs, will infuriate some Liberal Democrat MPs and peers who believe passionately that the EU should be enlarged to take in Turkey and more former Communist states.
Mr Davies said: "Reaching agreements, determining strong policies, making firm commitments - all these things become harder as the EU grows and more nations join."
He will call for reforms of the EU to beef up its efforts to become "a superpower" so that it can use its military strength for "good in the world".
"My fear is that the European Union is not living up to the potential we see for it, and that unless we make some changes it may never live up to that potential," he will say.
"I want the EU to be a superpower, an organisation with strength and authority able to speak with a single voice, and I want Britain to be at its heart."
Mr Davies's intervention coincides with a diluting of the Liberal Democrats' enthusiasm for Europe, which is now tempered by calls for reform of EU institutions and a greater role for MEPs. Although the Liberal Democrats remain the most pro- European party, the party's pre-manifesto document did not feature the European Union as a major issue for the election.
Nick Clegg, a former MEP who has been tipped as a future leadership contender, argued in a recent essay that "to be pro-European does not require an abandonment of basic critical faculties".
Next week party members will debate a motion in favour of a "yes" vote on the EU constitution, which is, however, expected to be passed without much dissent.
Yesterday the party published more details of its health policy, which included the scrapping of government targets for hospitals and GPs.
Schoolchildren would be taught about cookery and healthy eating in an attempt to cut obesity. Health funding would be earmarked through a "national health contribution", which would replace national insurance.Reuse content