In a 5-2 split decision, the justices have thrown out two provisions put into the law by the Conservative government's 2008 Tackling Violent Crime Act.
Those changes required that a defendant who raised a reasonable doubt about the functioning or operation of the breathalyzer machine also had to prove the reading was too high and that the driver had not consumed enough alcohol to produce it.
The justices say those last two provisions go too far.
They say once a reasonable doubt has been raised about the machine, that should be enough.
They add, however, that few defendants are going to be able to show the machine didn't work or the operator made a mistake.
The decision means that a defendant will still have to show that the breathalyzer machine wasn't working or that the operator didn't use it properly.
"In short, the accused might rely, for example, on a maintenance log that shows that the instrument was not maintained properly or on admissions by the technician that there had been erratic results, or he or she might argue that health problems had affected the functioning of the instrument," now-retired justice Marie Deschamps wrote for the majority.
She said it's a long-shot defence.
"It should be noted that the defence created by Parliament is not illusory simply because accused persons will rarely succeed in raising a reasonable doubt that the instrument was functioning or was operated properly. The existence of a defence must not be confused with how often those presenting it are successful."
The ruling applied to a Quebec case in which the accused raised a constitutional challenge to the drunk driving law. The defendant was convicted and chose not to appeal, but the Crown appealed for a ruling on the constitutional aspects of the case.
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