A recent Supreme Court of Canada ruling has prompted the Crown to drop a first-degree murder charge against a Nova Scotia man accused of killing his girlfriend.

Rhonda Wilson, a 31-year-old mother of three, was last seen on Aug. 7, 2002 when she left her Kentville home to go for a walk.

Her common-law partner, Albert Baird, reported her missing to police three days later, on August 10. The couple had two children together.

Baird was arrested in May 2013 and charged with Wilson’s murder.

However, the Crown has withdrawn the charge after concluding there was no realistic prospect of conviction. In making its decision, the Crown considered a recent Supreme Court of Canada decision on Mr. Big operations.

The high court noted several problems with the Mr. Big strategy in its July 31 ruling overturning the conviction of Nelson Hart in the deaths of his young daughters in Newfoundland in 2002.

Before his arrest, Baird was allegedly caught in a Mr. Big sting. However, the Supreme Court of Canada is calling on police to put tighter controls on the investigative technique.

During such operations, undercover police officers recruit a suspect to a fake criminal organization in order to obtain a confession about a prior crime or crimes. The top court noted that the technique is a Canadian invention and has been used hundreds of times by forces across the country.

The court did not call for an end to Mr. Big sting operations. However, it ruled in July that police must implement more stringent guidelines for their use.

Under the ruling, confessions obtained through a Mr. Big sting are inadmissible in court if they are not accompanied by corroborating evidence.

“So that meant that we had to re-evaluate our whole case and so when we did that we felt that there was no longer a realistic prospect of conviction,” says Crown attorney Bob Morrison.

Baird was released from custody after a hearing in Kentville provincial court. The case was in court for a status update before a preliminary inquiry, which was set to begin early next year.

Morrison says he informed Wilson's family before the charge was withdrawn, adding that some of her relatives were in court for Wednesday's proceedings.

"For us it was a difficult decision, but it is one that had to be made," he says. "For the family, they were looking for something to happen and I think they were disappointed."

In a statement, the RCMP say they “maintain that there were reasonable and probable grounds to lay the first-degree murder charge based on information gathered during the investigation.”

This is the first case in Nova Scotia to be impacted by the Supreme Court of Canada decision. The Crown says it is difficult to say whether murder charges could be dropped in other cases.

“I hope not, but I don’t really know,” says Morrison.

The Mr. Big strategy was used in the case of Penny Boudreau, who was convicted of killing her daughter Karissa Boudreau, and also in the case of Jason MacRae, who was convicted of killing his wife Paula Gallant.

Morrison says the Supreme Court ruling will not affect those cases because there was further evidence to support their confessions.

“I don’t see the Mr. Big, the undercover operation that they do, going away because they have been very effective and they have resolved a number of homicides across the country,” says Morrison.

Wilson’s body has never been found. Her case was added to the Nova Scotia Rewards for Major Unsolved Crimes program in 2010.

With files from CTV Atlantic's Kayla Hounsell and The Canadian Press