A prior restraint is an official government restriction of speech prior to publication. Prior restraints are viewed by the U.S. Supreme Court as “the most serious and the least tolerable infringement on First Amendment rights,” according to the Court’s 1976 opinion in Nebraska Press Association v. Stuart.1 Since 1931, the Court repeatedly has found that such attempts to censor the media are presumed unconstitutional.
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One interesting aspect of this area of the law is that while courts have been clear that prior restraints will rarely survive scrutiny even when national security concerns are raised, courts seem to be most willing to allow restraints when the administration of a trial is at issue, or when fair trial rights are implicated.
The First Amendment Handbook
by Reporters Committee for Freedom of the Press
5. Prior Restraints
http://www.rcfp.org/first-amendment-handbook/introduction-fair-trials-national-security-law-enforcement-investigations
Prior restraint (also referred to as prior censorship or pre-publication censorship) is censorship imposed, usually by a government, on expression before the expression actually takes place. An alternative is to allow the expression to take place and to take appropriate action afterward, if the expression is found to violate the law, regulations, or other rules.
Prior restraint prevents the censored material from being heard or distributed at all; other measures provide sanctions only after the offending material has been communicated, such as suits for slander or libel. In some countries (e.g., United States, Argentina) prior restraint by the government is forbidden, subject to certain exceptions, by a constitution.
Prior restraint can be effected in a number of ways. For example, the exhibition of works of art or a movie may require a license from a government authority (sometimes referred to a classification board or censorship board) before it can be published, and the failure or refusal to grant a license is a form of censorship as is the revoking of a license. It can also take the form of a legal injunction or government order prohibiting the publication of a specific document or subject. Sometimes, a government or other party becomes aware of a forthcoming publication on a particular subject and seeks to prevent it: to halt ongoing publication and prevent its resumption. These injunctions are considered prior restraint because potential future publications are stopped in advance.