Conservative leader David Cameron has set out his vision for the EU, saying it should focus on the "things that matter" rather than internal wrangling.
David Cameron wants more EU focus on globalisation
In a rare speech on European issues in Brussels, he resisted calls to lead a Tory retreat from the union.
He promised instead to push for the UK's national interest on issues like climate change and tackling poverty.
Europe in the 21st century needed "more flexibility, not more centralisation", Mr Cameron said.
Some Conservative MEPs are thought to be unhappy about the party leader's plan to leave the centre-right, federalist European People's Party (EPP) after the 2009 European elections.
He is also under pressure from euro-sceptics at Westminster, including a small group of MPs who want British withdrawal from the EU, who say he has broken his promise to pull out of the EPP sooner.
Mr Cameron is attempting to form a new group in the European Parliament, the Movement for European Reform.
Mr Cameron, who is trying to attract like-minded parties to join the new group, spoke at its first conference, which was also addressed by Czech Prime Minister Mirek Topolanek.
Under EU rules there must be parties from at least five countries to form a parliamentary group.
In his speech, Mr Cameron said the priorities should be the "three Gs" of globalisation, global warming and global poverty.
He reaffirmed his commitment to a referendum on the European Constitution.
'Debris of the past'
He said: "Those who will succeed in the 21st century will be those who can adapt, who can respond quickly, who can innovate.
"The modern world places a premium on diversity over uniformity. It forces a focus on results over procedures.
"The European Union needs to change if it is to be fit for the challenges of the new century, not stuck haggling over the debris of the last."
Mr Cameron added: "Only a decentralised political system will be able to hold Ireland and Turkey, Italy and Estonia in any sort of community
"Some people say that because we are 'widening' Europe we need to 'deepen' it too.
"But that doesn't make sense. Yes - of course we need a new framework to make a bigger EU work.
"But there is no case for the Constitution, or a Constitution-lite."
Europe minister Geoff Hoon said Mr Cameron's pledge to withdraw from the EPP - a move he said was described by senior Tory MP Ken Clarke as "head-banging" - would reduce Britain's influence in Europe.
"Unless David Cameron now names which other parties he has persuaded to sign up to his new group, his speech will be seen as nothing more than warm words," he added.
Liberal Democrat foreign affairs spokesman Michael Moore said Mr Cameron had isolated his party from Europe because he refused to accept the reality that European countries had to work with each other.
UKIP said Mr Cameron had won his party's leadership contest by promising to be "tough" on Europe in order to attract euro-sceptic supporters but had since let them down.
Commenting on Mr Cameron's speech, UKIP leader Nigel Farage said: "If you're in Europe, then you're run by Europe.
"This is the same Mr Cameron who deliberately misled not just his party, but the public, with a vow to withdraw Britain from the Social Chapter, when he knows full well that this is not possible unless Britain withdraws from the European Union."