But the Prime Minister stressed during question time when challenged by William Hague, the Conservative Party leader, that it was a sensible measure, because otherwise the option of joining the single currency would be removed altogether.
The Tory leader, demanding the exact figure, told the Prime Minister: "The Government is spending millions of pounds of taxpayers' money preparing to join the euro early in the next Parliament - you won't say how much has been spent.
"Yesterday the Home Secretary, Jack Straw, said in this House that `unnecessary secrecy in government has long been held to undermine good governance and my party has long been committed to change'.
"So, what is the ball-park figure of what's been spent so far - is it pounds 1 million, pounds 10 million, pounds 20 million, pounds 100 million?"
But Mr Blair insisted: "When we actually announced this plan we said what the cost would be. Therefore, people know how much we are going to spend on the changeover plan.
"It is sensible, because otherwise we would not be in a position either to be prepared for the euro - which is going to affect business in this country - nor would we be able to join the euro even if at a later stage the country voted for it," the Prime Minister said.