lo encontré en Ultralingua

mandato judicial

http://www.lectlaw.com/def/h001.htm
"A writ of habeas corpus is a judicial mandate to a prison official ordering that an inmate be brought to the court so it can be determined whether or not that person is imprisoned lawfully and whether or not he should be released from custody."

"Writ - Mandamiento.
Writ of attachment - Mandamiento de ejecución y embargo, mandamiento de embargo.
Writ of error - Recurso de casación.
Writ of execution - Mandamiento de ejecución y embargo, mandamiento de embargo.
Writ of mandamus - Orden judicial.
"
http://www.geocities.com/susanacr_99/legalw.htm

Todos ellos son exactos, pero no intercambiables. Depende del resto del texto. Sería útil que nos hicieras saber a qué tipo de acción se refiere

A mí entender el Parlamento estaría reconociendo (en tu contexto) una nueva acción. Así se llama a la acción jurídica a través de la cual se puede reclamar un derecho o declaración emanada de juez competente, ahora bien, a veces writ significa lo que en derecho romano es "escritura" pública. Te paso lo que dice el Webster's Revised Unabridged Dictionary (1913) (web1913)
2. (Law) An instrument in writing, under seal, in an epistolary form, issued from the proper authority, commanding the performance or nonperformance of some act by the person to whom it is directed; as, a writ of entry, of error, of execution, of injunction, of mandamus, of return, of summons, and the like.

Note: Writs are usually witnessed, or tested, in the name of the chief justice or principal judge of the court out of which they are issued; and those directed to a sheriff, or other ministerial officer, require him to return them on a day specified. In former English law and practice, writs in civil cases were either original or judicial; the former were issued out of the Court of Chancery, under the great seal, for the summoning of a defendant to appear, and were granted before the suit began and in order to begin the same; the latter were issued out of the court where the original was returned, after the suit was begun and during the pendency of it. Tomlins. Brande. Encyc. Brit. The term writ is supposed by Mr. Reeves to have been derived from the fact of these formul[ae] having always been expressed in writing, being, in this respect, distinguished from the other proceedings in the ancient action, which were conducted orally.

{Writ of account}, {Writ of capias}, etc. See under {Account}, {Capias}, etc.