Analysis By Nick Assinder BBC News Online Political Correspondent

Blair's statement is due at 1230 BST on Tuesday

He confessed he had changed his mind out of frustration that the arguments over the merits of the constitution were being lost amid the furore over his previous refusal to offer the public a vote.

It had, needless to say, absolutely nothing to do with any alleged threat by Rupert Murdoch to withdraw his support unless there was a vote.

And he confirmed that if the British people vote 'no' then it will mean precisely that. No, apparently, will mean no.

But that is where the clarity ended and the fog once again started to descend.

Unenlightening

For many, the key question is exactly what happens next in the event of a 'no' vote.

The prime minister was, at the same time, both brutally honest and unenlightening.

"I do not know what would happen," he said.

Clearly - and in apparent contradiction to what he had suggested during prime minister's questions on Wednesday - he could not simply tinker with the document and bring it back for another vote, he said.

"I cannot say anything more than we will have to come back to the [European] council and discuss the way forward."

But such a move would be "significant".

It certainly would.

It would be particularly significant for his leadership which would almost certainly be brought to an end with a 'no' vote.

'Way forward'

He even appeared to tacitly accept that. And, to that extent, what happens next would not be his problem.

His successor would be landed with the problem of going back to the other EU states and "finding a way forward".

Exactly what that way forward might be is no clearer today than it was yesterday or the day before.

Yet, almost in the same breath, the prime minister was a little more clear on what a 'no' vote would mean if the Tories got their way - it would lead to a fundamental review of Britain's relationship with the EU.

He did not say so in as many words, but the persistent suggestion is that it would lead to either withdrawal from the Union or some sort of associate membership.

Tightrope

But this is an increasingly wobbly tightrope for the prime minister to walk.

On the one hand he is suggesting rejection of the constitution would be serious, but would not force him into withdrawing from Europe.

On the other, he is suggesting the Tories want a 'no' vote so they can move deliberately towards withdrawal.

He was equally unenlightening about the basis on which the referendum campaign will be fought.

Bitter battle

Will it be about the fine detail of the constitution, or will it be a wider argument on whether Britain should be in or out of Europe?

What he did promise was that, unlike the yet to be witnessed pro-single currency campaign, he will be out there, leading the charge for the 'yes to the constitution' campaign.

It will be a bitter battle. Whether it will be an enlightening one is another question.