European Commission President Jose Manuel Barroso has called on the UK to lead a debate on how to make the European Union work better.
Mr Barroso knows that Tony Blair's days are numbered
Mr Barroso said the UK could "drive from the centre or sulk at the periphery", in a speech at the Royal Institute of International Affairs.
He asked Britons to move on from the for-and-against Europe debate and concentrate on how to improve the EU.
He stressed that the UK had already led the push for EU reform in many areas.
The UK spent much of its EU presidency last year pressing for a shake-up of the budget, and the Common Agricultural Policy.
Mr Barroso described the UK as a driving force for economic competitiveness in Europe, and highlighted its role in setting up the emissions trading scheme, promoting co-operation on counter-terrorism, and establishing energy security as a top priority.
Leader of debate
However, officials in Brussels have noted that Tony Blair's most likely successors - Chancellor Gordon Brown or Conservative leader David Cameron - do not share his enthusiasm for Europe.
Neither was mentioned by name, but the speech highlighted concerns in Brussels that the UK could in future ditch the Blair policy of being at the heart of Europe.
"The United Kingdom will always have influence in Europe - its size, its economic power and its international network will ensure that," Mr Barroso said in his speech.
"So the question is, does the UK want to shape a positive agenda, which reflects its own agenda, or be dragged along as a reluctant partner?
"Do you want to drive from the centre or sulk from the periphery?"
Costs and benefits
The speech came on the same day a poll was published indicating that most UK chief executives believe the costs of EU regulation outweigh the benefits of the single market.
But Mr Barroso also appealed to UK ambitions in other policy areas, beyond the economy.
"If the UK wants to tackle climate change, if it wants to fight poverty in Africa, if it wants to deliver greater external security, if it wants to have an open, competitive environment, then the UK needs the EU," he said.
"At the same time let us recognise another important truth: that the EU needs the UK."
He said that decision-making in Europe was "awful", it took too long and it was not transparent enough.