Mr Blair surrendered the initiative in October 1997 when he gave Gordon Brown control over the assessment. But this process is flawed. The Chancellor often points out that more work has been done on the assessment than on any previous economic decision. But outsiders will not see the 18 background studies until Mr Brown makes his Commons statement this month. The 2,000 pages of analysis will underpin a fait accompli rather than promote open debate. How many trees will have to be cut down? As Randolph Churchill said of an earlier “prudent” Chancellor: “The forest laments in order that Mr Gladstone may perspire.”

The five economic tests have assumed a mythological status, like the forbidden questions in a Wagner opera, or Clause Four in pre-new Labour days. As several witnesses told the Commons Treasury Committee during its recent inquiry, the insistence on a “clear and unambiguous” verdict is an impossible yardstick when the tests are inherently matters of judgment rather than measurement.

No one disputes that the economics is crucial. Ed Balls, the Treasury’s chief economic adviser, delivered a weighty lecture last autumn on the damage done in the past when far-reaching economic decisions were taken for immediate political reasons, ranging from the return to the Gold Standard in 1925 to the ill-fated entry into the European Monetary System in 1990. He was right. Going in at the wrong exchange rate, or when the economies are out of sync, would obviously be a mistake.

There are strong arguments against entry now: notably, the sluggish state of the German and other eurozone economies, and the over-rigid limits on borrowing in the stability and growth pact. These issues leave aside political factors such as the bitter divisions within the EU over the Iraq war, which would make it very hard to win a referendum in the short term.

Yet if not now, when? The Treasury’s assessment is passive, an inspector’s report. There is a sense of “take it or leave it”: if the Europeans do not measure up to our standards, we should leave the euro alone for another few years. If the verdict is “no, not yet”, it strains credibility to have a further review next year. As Sir Douglas Wass, a former Treasury Permanent Secretary, and two former chief economic advisers, argued this week: “The economic facts will not change swiftly to suit a political timetable.”

But that implies that the economic facts are clear-cut and a “not yet” decision will have no impact. The downside of staying out has been limited so far because of the relative strength of the British economy (seen by sceptics as cause and effect) and because overseas investors have regarded any delay as temporary. A persuasive report by a group of international economists, chaired by David Begg, of Imperial College Business School, argued that a negative assessment would result in lost foreign investment (already well down last year), reduced competitiveness and missed trade with the eurozone.

Moreover, the Begg report, commissioned by Britain in Europe, though unconnected to its campaigning, argues that we have already converged significantly with the eurozone. Britain may already be closer to many eurozone economies than they were to each other when the euro started, or are now. And as the eurozone continues to integrate, it may be harder for Britain to converge. Similarly, in so far as the fiscal and monetary arrangements of the eurozone will change over the next few years, Britain will lose its chance to affect their development.

None of this matters for those who regard joining the euro as an unacceptable surrender of sovereignty. From them, “not now” means “never”. But that is not what Mr Blair and Mr Brown have said. Nor is this the view of a majority of the public. According to the latest Populus poll for The Times, a third are in the never c a third oppose entry now but would not rule it out in future; and the remaining third favour entry now or as soon as the economic conditions are right.

The gap in the Government’s strategy is converting conditionality into achievement. That is why Mr Blair needs a more active approach, going beyond technical preparations for a new currency. Instead of just identifying obstacles to convergence, the Government should say how it intends to overcome them. Somewhat belatedly, Mr Brown announced in the Budget a study of housing finance. Encouraging people to borrow to buy houses at fixed long-term interest rates, as in most of Europe and the US, rather than variable short-term rates, is vital if entry is to work — and is desirable, in or out of the euro.

But, as the Begg report notes, conversion on the necessary scale would probably take two to three years to achieve. That would allow Mr Blair to set a public timetable of, say, three years or so for entry, as Robin Cook has argued. This would remove the doubts that a bald “not yet” conclusion will create, while allowing time for changes in policy and a referendum.

Some sceptics have seen Mr Blair’s willingness to shift responsibility, and blame, to the Treasury as a convenient cloak for his own reluctance to face such a controversial decision. Even if partly right in the first term, this is no longer true: hence Mr Blair’s anguished talks with Mr Brown to ensure that the Commons statement has a pro-euro gloss.

But even if, at Mr Blair’s insistence, Mr Brown does not formally rule out a further review, and a referendum, later this Parliament, few will take this seriously on its own. The “not yet” assessment will be widely, and rightly, seen as “no for a long time” unless Mr Blair produces a convincing timetable and date for entry.

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