The plans would prevent any member of staff in England with a substantive NHS contract from doing agency shifts at their own or any other NHS trust.
The RCN has condemned the lack of consultation over the ban and says members should be free to work in ways that suit them best.
NHS Improvement wrote to trusts last month advising them of the new arrangements but neither the RCN nor the NHS Staff Council agreed the move and there was no consultation.
The RCN will be advising reps to lodge disputes with trusts intending to prevent members from working in their own hospitals through agencies just because they have substantive NHS contracts.
Reps are encouraged to find out if their employers are implementing the ban and report back to their local RCN officer. The RCN will keep reps informed of further developments.