Ed Miliband is favourite, for the first time, to defeat his brother and be elected as leader of the Labour party.
Leading bookmakers shifted their odds to favour the younger Miliband following his late surge in the polls yesterday.
In what has been described by some commentators as a worst case scenario for the party, analysis of voting intentions by the Left Foot Forward website suggests David would top the poll on first preferences but go on to lose by 49% to 51% once the second, third and fourth preferences of the losing candidates were redistributed.
If correct, it would be a dramatic end to the five-month contest, delivering a devastating blow both personally and politically to David Miliband — the older brother who had long been considered the most likely victor.
The result will be announced in Manchester at 4.45pm as delegates gather for the party's conference.
In another family twist, Mr Miliband is thought to be considering both Ed Balls, another leadership rival, and his wife Yvette Cooper, the shadow work and pensions secretary, as his choice for shadow chancellor.