In a unanimous decision, the justices overturned a B.C. Court of Appeal ruling that ordered a new trial in the case.
They also upheld the $4 million in damages awarded to a boy and his mother after he was born brain damaged in May 2001.
The appeal court ordered the new trial because the judge lifted 321 paragraphs of his 368-paragraph decision straight from the written submissions of the boy's lawyers, mostly without attribution.
Chief Justice Beverley McLachlin, writing for the court, says that doesn't warrant a new trial in the case. Copying material does not, by itself, render a judgment unfair.
"I conclude that while it is desirable that judges express their conclusions in their own words, incorporating substantial amounts of material from submissions or other legal sources into reasons for judgment does not without more permit the decision to be set aside,"she wrote.
She said judicial copying is a long-standing and accepted practice.
"It may not be best practice for judges to bulk up their judgments with great swaths of borrowed material," she went on.
"But the fact remains that borrowed prose, attributed or otherwise, does not, without more, establish that the judge has failed to come to grips with the issues required to be decided."
She said legal rulings are often assembled from various sources, without attracting accusations of plagiarism.
"Judgments routinely incorporate phrases and paragraphs from a variety of sources, such as decided cases, legal treatises, pleadings, and arguments of the parties," she wrote.
"Appellate judges may incorporate paragraphs borrowed from another judge on the case or from a helpful law clerk. Often the sources are acknowledged, but often they are not.
"Whether acknowledged or not, they are an accepted part of the judgment-writing process and do not, without more, render the proceeding unfair."