Police are investigating after a number of Stena Line workers were found with drink and drugs in their system on the Belfast to Cairnryan ferry.
Up to 20 workers are reported to have been randomly tested after controlled drugs went missing from the ship's medical station.
Around five are understood to have been suspended, while others had prescription drugs in their system.
The incident is now being probed by Belfast Harbour Police as they have jurisdiction, but they would not reveal whether arrests have been made.
A spokesperson said: "The matter has been referred to Belfast Harbour Police and remains under investigation."
A Stena Line spokesperson added: "Stena Line can confirm that as a result of random testing, a number of employees onboard its Belfast – Cairnryan operation have failed the test and as a result Stena Line is now carrying out a full investigation as part of its established disciplinary process.
"We can confirm that it was identified through routine inspections onboard that controlled drugs were missing from the ships medical station that is in a secure location onboard the vessel, for this reason we carried out a drug and alcohol testing."
The Daily Record reported that the staff were immediately suspended but the ferry firm added it "will not comment publicly in detail on matters of employment", saying: "The police were notified immediately."
They went on to say they have a "strict policy in place prohibiting the use of drugs unless they are prescribed by a medical professional and do not inhibit the performance and ability of an employee to carry out their work in a safe way, the same policy contains the zero blood alcohol level requirement".
The company also said it carries out "a number of tests annually; including random and/or with cause, alcohol and drug tests across its business" to ensure the health, safety and wellbeing of its employees and customers.
Stena Line's spokesperson declined to reveal exactly when the incident took place.
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