Unions have said it is a “national disgrace” that workers dismantling old North Sea oil and gas rigs will be paid less than the minimum wage, despite new promises by ministers to protect people working in UK waters.

The government issued guidance on Thursday that it said would ensure seafarers, people who are based on vessels as opposed to fixed installations such as oil platforms, receive the minimum wage.

But the RMT union said the guidance only applied to UK waters out to a 12-mile limit, which would not cover most oil and gas activity.

One firm decommissioning the BP Miller platform had workers on 12-hour shifts for $45, or about £2.70 an hour, the union said. A second contractor reportedly paid similar rates on Canadian Natural Resources’ Murchison platform. The minimum wage is £7.50 for over-25s.

The concern is international crews are being paid unfairly low rates and undercutting opportunities for UK workers.

“This is an issue which to my mind is a national disgrace,” said Jake Molloy, a regional organiser for the RMT. “This change [today] will do nothing to protect our members, or the exploitation of foreign nationals by the oil and gas industry, nothing,” he said.

Labour said it was deplorable that workers were not getting the minimum wage.

Rebecca Long-Bailey, the shadow business secretary, said: “The government hasn’t done enough to stop employers denying workers their rightful pay, which is grossly unfair to workers but also unfair to the businesses that do play by the rules.”

Under plans published on Thursday, UK Border Force will issue information to seafarers on their rights, in more than 50 languages.

Andrew Griffiths, a business minister, said: “Today we are making it crystal clear that if you work in UK waters you are entitled to at least the minimum wage and all employers – no matter where they’re from – must pay it.”

Saipem and Heerema Marine Contractors, which are carrying out the BP and Canadian Natural Resources rig work, said they paid in line with rules laid down by the International Transport Workers’ Federation.

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