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No speech by the prime minister has been agonised over for so long. None will be of such consequence. None will be based on a bigger gamble.

Today could mark a first step on a road leading to Britain's exit from Europe 45 years after we first joined. David Cameron couldn't have been clearer that that is not his intention nor his belief about what will happen next. However, what he declared today was that he was no longer in charge. The people are to have the final say.

Even without the in-out referendum pledge this would have been a significant speech.

It was a plea for Europe to change itself for its own sake - abandoning the shared goal of "ever-closer union", accepting that the single market rather than the single currency was the key shared ambition and agreeing that powers should be returned to member states not inexorably transferred to Brussels.

It was also a plea to Europe to change so that the UK could stay a member and settle the issue that has caused so many prime ministers so many problems.

Beyond the call for "a more flexible, adaptable and open European Union" there was very little about what changes the prime minister will argue for. In other words we do not know what the new settlement he is seeking to negotiate would look like.

David Cameron says that if he can secure it he will campaign heart and soul for a yes (ie: In ) vote. He was asked whether that meant he would vote No if he didn't get what he wanted - whatever that is. He dodged the question. I suggested to him that he wanted people to believe that's what he would do but was scared to say so. He dodged the question again.

This is a gamble just like the one he took when making his "big, open and generous" offer to form a coalition in the aftermath of his failure to win the 2010 election outright.

He is gambling that his referendum promise will calm rather than stir the fury of Eurosceptics both inside and outside his party, that he can persuade 26 other European leaders to give the UK the deal he wants and that voters will then choose to back it.

If he pulls it off he will restore party unity, see off the threat of UKIP, put Labour on the back foot and secure a relationship with the EU which is no longer a political nightmare for him and his party.

If he doesn't the name Cameron will be added to those of Wilson, Thatcher and Major - those whose premierships were destroyed by that most toxic issue in politics - Europe.