The news the government is set to announce European referendum plans has sparked a range of reactions.
Frank Field MP says Mr Blair will win support for consulting the public
Chris Bryant MP, chairman of the Labour Movement for Europe, said his support for the referendum would depend on its remit.
He told the BBC: "If we're going to have a wide-ranging debate about Britain's role in Europe and our membership of the European Union then yes.
"And perhaps if we also had a referendum on the wider issue and, who knows, a vote on whether Britain should be joining the eurozone then I'm certainly up for that. I'm sure that Britain will say yes."
While Mr Blair will be hoping the U-turn wrong-foots the Conservatives, Labour MP Frank Field, who is part of the Vote 2004 campaign that has argued for a referendum, says the government will lose the battle to come.
"While I think the prime minister will lose the vote, I think he will gain incredible support in the country that he has in a sense found his reverse gear and is willing to consult the people in this decision," he told the BBC.
But Labour's North West England MEP Gary Titley said he was disappointed the referendum looked certain to go ahead.
"I think what you have got here is a sort of populist campaign, whipped up by certain sections of the press, that bears no resemblance to the facts."
He said it would be difficult to have a proper debate on the issue given the "distortion" of the facts being published.
"And where do you stop? What will happen in the future is that every time somebody is against something they will call for a referendum. The more referenda you have the harder it is to resist that call."
Eurosceptic Labour MP Ian Davidson - who is anti-constitution and pro-referendum - said he was still likely to oppose the constitution as it went through parliament for approval.
The Glasgow Pollock MP - who this month launched the campaign group Labour against a Superstate - said he was "absolutely delighted" about the prospect of a poll.
He said he could "see the merits" of not tying up parliament with the referendum in the run-up to the next general election.
Scottish National Party leader Alex Salmond welcomed the apparent U-turn but said it must be must be "more than a mere ploy to get Labour though the next election".
"Tony Blair has his work cut out between now and June in getting
the constitution into a form that is supportable from a Scottish point of
view," he said.
He also hopes European fishing laws which affect Scotland would be revisited.
Gisela Stuart MP, who represented Labour on the convention drawing up the constitution, told the BBC the referendum would bring the European debate "out in the open" with all parties forced to "flesh out and debate their cases".
"I think it would be a much more honest debate and I think the electorate would welcome that", she added.
Echoing Ms Stuart's comments, Labour peer Lord Haskins said he was pleased the vote would widen the debate on Europe.
"It will be the base for really good national debate about Britain in Europe," he said.
He called for the referendum to be held on the same day as the general election.