Union leaders and politicians have demanded action following news that some crewmembers on Northern Isles cargo boats were being paid less than £4 an hour.
The RMT Union has blasted the hourly rate for staff onboard the Hildasay and Helliar as “poverty pay”.
It has launched an online petition calling for all seafarers on Scottish government contracts to be given at least the minimum wage.
Although the Scottish government contract was awarded to Serco back in 2012, the running of the cargo boats has been outsourced to Seatruck, based in Heysham, which pays the crews onboard the vessels.
Seatruck says applying the national minimum wage would place it at a serious disadvantage with its competitors. It says that, under current legislation, the national minimum wage is not applicable to the crew of the vessels, adding that the Estonian workers involved are deemed not to be ordinarily working in the UK.
RMT spokesman Gordon Martin said: “The most up-to-date figures we have is they are getting paid £3.66 per hour.
“This is a Scottish government contract, the Northern Isles ferry service, and it is totally unacceptable that these workers from Estonia are being paid way below the UK national minimum wage.”
Scottish transport minister Humza Yousaf said: “That’s an unacceptable situation. Transport Scotland will be in touch with Serco. Serco will offer to pay the national minimum wage, and backdate that on behalf of the company.
“That offer I would urge the company, Seatruck, to take up. I will also write to the UK shipping minister to say they should impose that minimum wage on non-UK nationals, regardless of whether they are working in international waters or not if they are working on UK contracts.”
Mr Martin welcomed Mr Yousaf’s comments.
“But I would qualify that by saying this has been raised with Derek Mackay, who was the transport minister before Humza Yousaf, and indeed with Keith Brown, who was the transport minister before Derek Mackay.
“We’ve had warm words and no action. This time we expect action, and that action must be to the satisfaction of us at the RMT that the matter has been dealt with properly.
“If the matter hasn’t been dealt with properly we are going to campaign, and continue to campaign, to get justice for the seafarers involved.”
The Scottish government says Serco NorthLink was initially unaware that workers were being paid below the UK minimum wage. It brought it to the attention of Transport Scotland as soon as the unions raised the issue.
Officials say Serco NorthLink has already made offers to Seatruck to bring the pay of crew up to minimum wage level. But Seatruck has not accepted the offers.
A Transport Scotland spokesman said: “Scottish ministers are strongly of the view that all workers employed on Scottish government funded contracts – including through sub contracts – should be paid the national minimal wage, if not the living wage, and deem the current situation with Seatruck unacceptable.
“The minister for transport and the islands will ask Serco NorthLink Ferries to write to Seatruck a further time, repeating the offer to fund the gap to bring the pay of crew members up to the national minimum wage, and he urges the company to accept that offer. The minister will also write to the UK government to ask them to take action on this.
“Ministers are currently considering all available options to strengthen future ferry contracts to include a formal requirement for the national minimum wage, and preferably the living wage, to be paid to all employees.”