Gordon Brown needs to match his warm pro-euro words with action to prepare for joining, Liberal Democrat Treasury spokesman Matthew Taylor has said.
Matthew Taylor says the UK has already lost out from staying out
Mr Taylor appealed for an end of the government's indecision on the euro, saying the UK had already lost out by failing to join.
After six years of supporting euro entry in principle, Prime Minister Tony Blair was still as much in the dark as everybody else as to when there would be a referendum, he said.
"The time for indecision is over, warm words are not enough," said the pro-euro Mr Taylor.
The Lib Dem spokesman said the chancellor had sounded supportive of the euro during his last major euro statement, back in 1997.
He feared the same pattern of public enthusiasm without specific preparations for joining risked being repeated.
Mr Brown was "saying no but dressing it up in a warm kind of almost yes, as if we are poised any moment to join", said Mr Taylor.
He wanted to know whether ministers were going to get on with negotiating the terms for British entry, particularly on the exchange rate.
"The question, after hearing all the chancellor had to say, is if he wants the convergence and the flexibility he talks about, is he going to take the action to get it?
"Today the chancellor's warm words suggest 'yes' and we welcome that. But staying out of the euro for so long has already had its costs."
The UK's reduced share of investment within Europe, a collapse in manufacturing industry and a "rip off Britain" culture were evidence of the damage failing to join had caused, he said.
Mr Taylor suggested three of the government's five economic tests - on the impact on the City, jobs and investment - had clearly been met already.
And he pointed to the benefits highlighted in the Treasury's 18 studies.
Adopting the single currency could bring more growth and more jobs, as well as reducing costs of the average mortgage by £50 a month, he argued.
There would also be an end to "rip-off Britain" culture, he predicted, saying that even chips were cheaper in France and Germany.