"Giving a speech on Europe in Britain is like turning up for a 100 metres athletics final, looking up and realising that you have actually been entered for the 100 metres high hurdles."

But he urged Britain to lead the debate in Europe rather than taking part from the sidelines.

"Europe is not full of hidden plots. The more the UK leads the debate, the more you will get out of Europe. Europe is an opportunity not a threat."

He also made the case for the EU's proposed Reform Treaty - the subject of heated debate in Britain.

"We need to modernise our institutions so that they are more democratic and have more coherence externally. We cannot operate as 27 with a system created for 6."

Touching on the nerve of the debate in the UK, the commission president said the Reform Treaty "is not" the constitution.

At the moment the UK is locked in an internal political tussle on whether there will be a referendum on the new treaty, with the rest of Europe watching closely.

Opposition Conservatives are trying to push Labour prime minister Gordon Brown into holding the poll - he has so far refused, saying the treaty is sufficiently different to the original constitution, on which the government had previously promised a referendum.

The Liberal Democrats last week eased the pressure somewhat on Mr Brown, when leader Menzies Campbell said he did not back a referendum on the treaty. He later said there could be a case for a referendum on Britain's membership of the EU.

Mr Barroso's speech was given a standing ovation. However, some pointed out that he was already preaching to the converted - with the liberals traditionally pro-European.

"His speech is far more needed elsewhere, either at the Tories, if they are prepared to give a fair wind to a positive message from Brussels, or at Labour, with its doubts and hesitations over making a pro-EU case," Peter Riddell, a political commentator in The Times, wrote.