Pressure is mounting on Chancellor Alistair Darling over tax changes which could hit thousands of offshore workers.
A maritime union is protesting to UK ministers over the “vindictive and damaging” move, while Aberdeen South MP Anne Begg wants a meeting with Mr Darling.
Nautilus, which has 18,000 members, said new guidance from HM Revenue and Customs meant some vessels doing offshore work can no longer be classed as ships.
The change affects workers in the dive support and remotely operated vehicle industries, who may no longer qualify for seafaring income tax concessions.
The HMRC says it is duty-bound to apply the law.
Nautilus warned that “thousands” of North Sea workers would be hit by the tax change – particularly in Scotland – and has written to Mr Darling and Shipping Minister Jim Fitzpatrick outlining concerns.
A spokesman for the union said: “It’s an absolute outrage. This isn’t the first time HMRC has messed around with it. It’s incredibly damaging.”
The union said the existing settlement was arranged in 1991 to safeguard British employment against cheaper foreign competition. “It’s there for a very good reason,” the spokesman added. “These are people who spend a lot of time outside of the country having to compete against low-cost foreign labour.
“This erosion of the concession, we believe, is going to be potentially devastating for the UK offshore skills base.
“We’ve made high-level protests to the chancellor and the shipping minister.
“They keep doing it – it’s vindictive and damaging.”
Miss Begg has also written to the chancellor, requesting a meeting so she can ask that the UK Government seriously reconsider the implications of the decision.
The MP has been contacted by constituents who work in the offshore oil and gas industry in both the North Sea and around the world after they were told by HMRC that the vessel they work on no longer qualifies under the Seafarers’ Earnings Deduction scheme. This means they cannot claim a 100% deduction from income tax.
She said: “To add to the frustrations, the tax liability has been backdated to April 2007.
“I do not believe that the original legislation intended that these workers should be penalised in this way and I have, therefore, written to the chancellor, requesting a meeting in order that I can raise my concerns direct.”
An HMRC spokesman said: “Revenue and Customs need to make sure the law is applied correctly – this matter went before the special commissioners.” The special commissioners are independent and are brought in to make a decision when HMRC guidance is disputed.