The Supreme Court has declared prosecutors can renege on a plea bargain that would have allowed an Alberta woman to plead to careless driving after initially being charged with impaired driving in a crash that killed a couple and injured their son.

The Supreme Court of Canada has ruled that prosecutors can renege on a plea bargain that would have allowed a woman to plead to careless driving in a deadly crash. Canadian Press

The court was unanimous in its decision, the first time the justices have ruled on the discretion allowed prosecutors in plea agreements.

The case involved Olga Maria Nixon of Stony Plain, Alta., who was first charged with impaired driving causing death and injury in the 2006 crash. Nixon drove her motor home through an intersection near Mundare north of Edmonton.

Her blood-alcohol level was triple the legal limit.

A prosecutor concluded there were problems with the admissibility of the breath samples and agreed to reduce the charges to dangerous driving causing death and injury.

The prosecutor was also concerned that eyewitness evidence of the motor home driving erratically was not strong.

Further negotiation led to a deal for a plea of careless driving, a traffic offence carrying an $1,800 fine.

But the deal was quashed by senior Alberta justice officials, which made Nixon's lawyers cry foul.

The Supreme Court has never ruled on whether a prosecutor can cancel a plea deal. But it has looked at other cases involving the discretion of prosecutors.

"So long as the proceedings are ongoing, the Crown may be required to make further decisions about whether the prosecution should be continued and, if so, in respect of what charges," Supreme Court Justice Louise Charron wrote for the majority.