His actor sister Sarah announced on social media that he had passed away unexpectedly at home on Sunday.
He was "gone much too soon," according to his family, who also called him a "wonderful and passionate actor."
McCardie leaves behind a remarkable legacy that will be deeply missed.
McCardie, despite appearing in minimal roles in "Line of Duty" for the first two seasons, delivered a notable performance.
McCardie portrayed Alasdair MacGregor, the brother of the title character, played by Liam Neeson in Rob Roy.
He starred in various TV series, such as Titanic, Murder is Easy, and Dog Days, as well as in Agatha Christie adaptation.
Brian's off-screen persona was notably different from the character he played, he was warm, funny, and charming.
McCardie recently starred in the Outlander prequel "Blood of My Blood."
Line Of Duty producers Jed Mercurio and Simon Heath expressed their deep sadness over the death of Brian McCardie.