Offshore unions are preparing to "name and shame" oil companies they claim are blacklisting workers.
The unions believe NRBs are used to silence dissent on the rigs
But industry leaders insisted the practise of issuing NRBs - which stands for "not required back" - is not widespread on North Sea installations.
Workers labelled as NRB would not only not be required back by that particular employer, but would be unlikely to be wanted by any other operator.
The OILC and RMT unions said they would expose companies adopting the practise.
Jake Molloy, general secretary of the OILC, said the issuing of NRBs to workers employed through contractors was often unofficial and difficult to fight against in law.
He claimed some of the reasons given for workers being NRBed had previously included complaining or being too proactive about safety or being suspected of giving a tip-off to the Health and Safety Executive (HSE).
Mr Molloy said: "It is essentially a process whereby the owner or the operator of an installation can enforce a contractual obligation on the contractor employer to remove any individual from an installation for virtually no reason or any reason they choose to pick."
A spokesman for UK Oil and Gas, which represents offshore companies, said for security reasons it was important an operator has control of who has access to their installations, but said there was no evidence to suggest the practice of NRBs is widespread.
The spokesman said none of its members would compromise safety by taking action against workers who had raised safety issues.
But Ian Whewell, head of the Health and Safety Executive's offshore division, said he was concerned the industry was not facing up to the reality that NRBs do exist.
He added: "The nature of many contractor relationships is that if their staff are told they are not required back by management of the employing company they are unwilling to challenge the decision for fear of jeopardising the contractual relationship.
"There is also a fear amongst the employees involved that standing up for themselves may jeopardise their future employment offshore. This conceals the reality of NRB."
Mr Whewell said that because the use of NRBs was part of an employment contract there was nothing the HSE could do directly to tackle them.
But he claimed it was vital that workers could be confident that they would not suffer if they spoke out on safety issues.
"In HSE's view the apparent lack of natural justice for employees involved in NRB situations strikes at the heart of the creation of the sort of positive constructive safety culture necessary for delivery of the standards of health and safety essential in such a high hazard industry," he said.