Blair's speeches have reignited the euro debate
Ian Taylor, who is also a pro-European Conservative MP, said Mr Blair's latest speech on Europe means the question now is when a euro vote will be held.
That has not stopped pro-Europeans welcoming the comments as a new signal of euro enthusiasm.
Nor has it stemmed criticism from Eurosceptics, whom Mr Blair attacked in the speech in Birmingham.
Mr Taylor told BBC Radio 4's World At One the position had changed.
"It is now impossible for Tony Blair to be credible unless he has a referendum, so we are going to have a referendum. When, I don't know."
More work needed
But he wanted the prime minister to go further and manage the economy so the UK could meet the economic tests the government had laid down.
His words echoed the comments of Simon Murphy, Labour leader in the European Parliament, who said the "between the lines" message of the speech was that the UK's decision on the euro was "inevitable".
Conservative Party chairman David Davis criticised the timing of the speech, coming as it does at a time of international crisis.
"Why is he going for this distraction at this point in time," asked Mr Davis.
The Tory chairman argued Mr Blair was wasting time on the issue when there were "desperate problems" in schools, hospitals and policing.
Mr Davis suggested the government wanted to persuade the British public that joining the euro was inevitable when it was not.
There was a very different reaction from Liberal Democrat leader Charles Kennedy, who described the speech as one of the most significant from a British prime minister in recent years.
One had to go back to Ted Heath in the 1970s to find a UK leader espousing similar arguments for engagement in Europe, Mr Kennedy told BBC Radio 4's PM programme.
"This are very, very significant statements and I think this is a bit of a ground clearing exercise for the campaign that will follow in the course of this Parliament," he continued.
The Sun newspaper has suggested the prime minister is planning to hold a euro referendum in 2005 - a claim denied by Downing Street.
Mr Kennedy instead believed Mr Blair, who again stressed the need for key economic tests to be met before a vote can be taken, would plump for an earlier date.
Mr Blair's speech coincides with the publication of a new opinion poll, commissioned by pro-euro group Britain in Europe, on public attitudes to Europe.
Poll views
In the NOP telephone poll, 72% of those questioned said they supported European co-operation on defence.
And 58% thought Europe had a role in helping Britain secure currency stability.
Britain in Europe campaign director Simon Buckby said the poll showed British people shared Mr Blair's view that international cooperation was the best way to tackle many problems.
But the No Campaign's director, Dominic Cummings, said ICM polling for the group since the US terror attacks showed opposition to the euro might not be getting wider but it was hardening.
Dismissing Mr Blair's speech, he said people believed by a 59% to 22% margin that the 11 September events made the case against the euro entry stronger.
That view was shown too among those undecided on the issue because, Mr Cummings argued, they wanted to avoid "chaos" at a time of danger.