The prime minister used a speech in Cardiff yesterday to reaffirm his commitment to joining the single currency if the economic conditions are met.

He said: "On the euro, we should of course join if the economic conditions are right. A single currency with a single market for Europe makes economic sense."

He added: "The problem with hanging back... is that in the end, Europe does move on and the choice is then to remain a straggler, drop out altogether or to catch up. Hanging back rarely results in us not participating finally, it just delays it so the participation is on terms set by others. And often this has meant less favourable terms."

Writing in today's Times, Peter Riddell says: "Tony Blair wants Britain to play - and be seen to play - a constructive role in the debate over the future of Europe, rather than its familiar defensive posture. But there is a big gap between good intentions and winning the argument."