Foreign Secretary David Miliband has suggested the European Union should work towards including Russia, Middle Eastern and North African countries.
He said enlargement was "our most powerful tool" for extending stability.
In his first major speech on the UK's relationship with Europe, he said the EU would not become a "superpower" but should be a "role model" for the world.
It could be a "model power of regional co-operation" dedicated to free trade, the environment and tackling extremism.
He said the EU must "keep our promises to Turkey", adding: "If we fail.... it will signal a deep and dangerous divide between east and west.
"Beyond that we must keep the door open, retaining the incentive for change and the prospect of membership provides."
Mr Miliband made his address at the College of Europe in Bruges, Belgium, where Baroness Thatcher delivered her famous w
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