A decision is to be given on the Government's High Court bid to obtain a permanent ban on industrial action by prison officers.
The Ministry of Justice's application follows a move in February by the POA - the Professional Trades Union for Prison, Correctional and Secure Psychiatric Workers - to call on its members to take action short of a strike.
That led to a judge granting an interim injunction requiring the POA to withdraw a circular instructing members to refuse to undertake certain voluntary tasks.
The ruling in London on Wednesday follows a hearing earlier this month when Daniel Stilitz QC told Mr Justice Jay that the case was brought "to ensure that POA does not seek to breach the law again".
He said that, in view of the importance of maintaining discipline and security within the prison service, Parliament had enacted a prohibition on inducing prison officers to take industrial action under the Criminal Justice and Public Order Act 1994.
Despite this, he added, the POA had recently called or threatened to call industrial action on a number of occasions.
"Any inducement which leads prison officers to cease to provide services which they otherwise would have done is unlawful," said counsel.
He said the history of "unlawful" industrial action, together with the "inflammatory rhetoric"used by the POA, suggested a continuing willingness to call for industrial action despite Parliament having "clearly and deliberately" outlawed it.
"The Secretary of State is concerned that, unless the position is determined definitively by the court, there will be renewed calls for unlawful industrial action in the future.
"It is, therefore, critical to safeguarding the proper and safe functioning of the prison service that the scope of the statutory prohibition is confirmed."
John Hendy QC, for the POA, said it contended that each of the activities referred to in the circular were voluntary in nature - the officers were not obliged contractually to undertake them.
"Prison officers are, as a matter of goodwill, free to decide to volunteer to undertake the tasks.
"Likewise, once having volunteered, they are at liberty to decline to continue to volunteer. The POA denies that the circular constitutes unlawful industrial action."
He said steps were taken to ensure members fully complied with the February order and the court was being asked to consider the lawfulness of a circular which was no longer extant and never implemented.
There was no evidence that the POA had asked any of its 35,000 members to disobey any lawful instruction, he added.