The phrase "declared illegal" implies an action taken by a governing body or authority that deems an act or behavior as violating a law or regulation. The antonyms to "declared illegal" would be "authorized," "legitimate," "legal," "allowed," "permitted," "sanctioned," and "lawful." These words signify the opposite of illegal and suggest that an act or behavior is endorsed by legal authorities or institutions. Furthermore, these antonyms refer to acts and behaviors that comply with rules and regulations established and recognized by the government or relevant authorities. Overall, the antonyms signify that an act or behavior is within the boundaries of the law and therefore acceptable in society.