B-1, r. 3.1 - Code of Professional Conduct of Lawyers

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27. A lawyer must act pursuant to a mandate given to him by a client or when he has been designated to act for a client by a competent authority.
He may also act within the scope of a mandate given to him by another lawyer on behalf of a client, in which case the other lawyer’s client is also considered to be his client for purposes of this code.
O.C. 129-2015, s. 27.