Foreign sailors working in Scotland's oil industry are being paid as little as £2 an hour, it has been claimed.
The International Transport Workers' Federation (ITF) says at times more than half of the non-UK vessels in the North Sea employ crew on less than the minimum wage of £6.70 an hour.
Others are not paid at all, with more than £1.1m of overdue wages owed to foreign seafarers involved in the oil sector this year, documents seen by STV News show.
Ken Fleming, Britain and Ireland coordinator for the ITF, said: "For some reason, if you work in the UK and your employment happens to float, you're considered different than someone on land.
"But it's not a question of being on land - it's a question of being under the jurisdiction of the UK."
Foreign vessels are contracted to North Sea oil and gas firms through shipbrokers based in Britain.
However, under UK law it is legal to pay crew less than the minimum wage if they are recruited overseas.
Jonathan Roberts from the UK Chamber of Shipping said: "Seafarers are paid salaries commensurate with the country in which they live. This reflects differences in the cost of living in each country."
Sailors aboard a third of the 17 non-UK North Sea vessels inspected by the ITF in Scotland this year were owed money.
The crew of the Malaviya Seven, which is currently detained in Aberdeen, are due nearly £80,000 and less than a third are paid the minimum wage, the ITF says.
BP contracted the vessel for two weeks in June.
A spokeswoman for the company said: "During periods of peak demand, we need to contract ad-hoc vessels on short-term hire - this was the case with the Malaviya Seven.
"Despite our best endeavours to ensure the correct level of due diligence was carried out, in this instance, with hindsight, there was more we could have done.
"Acting on this, and recognising that we will always on occasion need to use ad-hoc vessels, we have worked with our vessel broker to create a list of pre-verified operators who have been subject to robust due diligence.
"This includes a new screening process to identify and manage human rights risks."
Owner GOL Offshore, which is based in India, has not responded to repeated requests for comment.
Northern Isles MP Alistair Carmichael is calling for a debate on the issue in parliament.
He said: "Unfortunately there is a history of this in the maritime industry - for most people what happens at sea is out of sight and out of mind.
"I strongly suspect that in the North Sea there has been an element of wilful blindness on the part of the industry.
"I think they've known what's been going on and I think they're pushing a lot of their costs on to shipping companies as they try to reduce their cost base."
He added: "If you have a genuinely level playing field it's in the interests of British companies."
It is possible for foreign seafarers to apply for minimum wage protection if they can prove that they have a strong connection to the UK.
However, the ITF says that extending existing legislation to everyone working in the North Sea automatically would improve conditions, create jobs, increase tax returns, and make the industry more competitive.
Shahid Khan, vice-chairman of the Glasgow-based human rights organisation Global Minorities Alliance, said: "It is deeply worrying that the multi-billion oil industry is engaged in the murky and shadowy practice of exploiting foreign workers.
"Such practices in the long run harm British labour market as well they put foreign workers in extreme vulnerable situation."
Oil and Gas UK, which represents Britain's offshore industry, did not respond to requests for comment.
Members of the RMT union held a demonstration in Aberdeen on Friday over what they described as the exploitation of foreign workers aboard Scottish Government-contracted ferries.
Northlink, which sub-contracted the ferry route between Aberdeen and Shetland to Seatruck, said it has repeatedly offered to cover the cost of paying the basic rate.
Seatruck claims paying crews the minimum wage would "distort its fleet-wide pay structure".