The chancellor is expected to say conditions are not right for euro
European Commissioner and former Conservative cabinet minister Chris Patten has made a scathing attack on the prime minister's approach to joining the euro.
The government is due to announce by 7 June whether or not it will hold a referendum on the single European currency.
Mr Patten is accusing the prime minister of failing to show leadership on the euro debate and failing to exert his authority over Chancellor Gordon Brown.
He told BBC Radio 4's Today programme that the idea that Mr Brown's five tests on euro entry represented "absolute economic truth" was "drivel".
Politics came into play too in the decision, said Mr Patten, who said Mr Blair was putting British influence in Europe at risk.
There were crucial arguments to be tackled in Europe, including economic reform, the EU's future shape and relations with America, he continued.
Patten says Blair is putting UK influence at risk
"Mr Blair's strength in putting those arguments will to a degree be hollowed out if Britain is felt to be semi-detached from the overall European enterprise and if Mr Blair is thought to be rolled over by Mr Brown," he said.
Writing in the Times newspaper, Mr Patten said the five tests insulted the public's intelligence and were the "institutionalisation of the chancellor's veto over the prime minister's hopes and the country's interest".
The Treasury was behaving "as though the
rest of Europe's economy is not yet worthy of us", said Mr Patten.
"By delaying, we continue to exclude ourselves from key decisions
about the eurozone's future - reform of the European Central Bank, for example, and the regulation of Europe's financial markets.
The decision is first of all 'if' and then it may be 'when'
"So long as we hang back from full and unequivocal EU membership, how can we convince them (EU partners) that, for all our patronising prevarication, deep down we are mad about Europe?"
Foreign Secretary Jack Straw instead said a lack of proper economic tests had spelled trouble when the UK joined the European Exchange Rate Mechanism (ERM).
Mr Straw said: "The setting of these tests is very sensible because we have made what amounts to a political decision ... about the rightness in principle of joining the euro, but what it all depends on is whether the economic circumstances are right."
If or when?
On Sunday, Commons Leader John Reid said: "The decision we are taking is not whether we will join the euro but it is when we will join the euro."
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But questioned on Tuesday, Mr Straw, seen as one of the ministers more cautious about euro entry, said: "It's first of all 'if' and then it may be 'when'."
It is widely expected the verdict will be "No, not yet" to entry when the chancellor delivers his assessment on the five economic tests.
But in a sign of continued wrangling, Downing Street this week said Mr Brown and Mr Blair were having "detailed discussions" about the decision.
Labour peer Lord Simon of Highbury, vice-chairman of pro-euro campaign group Britain in Europe, believes the tests have been met.
The former trade minister told the Daily Telegraph newspaper: "Uncertainty from here on in will be much more damaging than it has been for the last two years."
"I know companies are looking at investment decisions now and if they don't get a positive decision they will not invest in this country."
Mr Patten's outburst reflects a growing exasperation of pro-euro campaigners with government's hesitant approach to joining.
Their hopes now are concentrated on Mr Blair ensuring a referendum is held before the next general election.
But James Frayne, campaign manager of the No Campaign, said: "Pro-euro hardliners like Patten have become so obsessed with the euro that they've lost touch with economic reality.
"The economics are clearly wrong - the Eurozone is suffering from massive unemployment and slow growth and Britain is doing much better outside.
"Joining the euro now when two thirds of businesses and the public are against it would be a huge mistake."