At the time, there were accusations of cheating on both sides, and Aubry has made no secret of her contempt for Royal.
Despite being expected to withhold backing from both remaining candidates, Royal said she was supporting her former partner because he was best placed to win by a convincing margin in this Sunday's run-off and go on to defeat President Nicolas Sarkozy in the presidential election next year.
"France faces a decisive moment in its history," she said.
"We on the left don't have the right to mess up this appointment with the people of France, who expect us to be united in their service."
Hollande described his former partner's decision as "elegant and responsible".
He said that she, more than anyone, knew the importance of party unity - a reference to the in-fighting and personal hatreds that weakened her campaign in 2007.
In the first round of the primary, Hollande topped the poll with 39.2 per cent and Aubry was second with 30.4 per cent.
A poll by Opinionway on Tuesday suggested that Hollande would win on Sunday by 54 per cent to 46 per cent.
More than 60 per cent of Royal's supporters told pollsters that they would switch their vote to Aubry.
It remains to be seen whether her endorsement will change their minds.
The outcome of the primary will also depend partly on the 17 per cent who voted for Arnaud Montebourg, the most left-wing candidate.
- Independent