Leaders of the RMT Rail and Maritime Union have taken the decision because they believe that seafarers from abroad, particularly those from eastern Europe, have taken jobs that would have gone to British nationals.
Union representatives argue that companies operating out of ports in the UK pay foreigners less because they are used to a lower standard of living and because they know that, in most cases, the wages will be tax-free.
It is thought to be the first time that a union has contemplated such action on a systematic basis, although individual workers have often taken the initiative.
It is possible that unions in other industries, such as construction, could now follow the RMT's example. Building sites, especially in south-east England, have seen a significant influx of foreign labour since the Iron Curtain was removed. Members of the seafarers' union will be urged to pass on to the Inland Revenue the names of vessels and those of the employers concerned.
The executive council of RMT has noted 'with alarm' that senior tax collection officers have admitted that foreign seafarers invariably did not have United Kingdom tax deducted from their wages. This was because the Inland Revenue relied on the co-operation of taxpayers, and their employers, to make the deduction.
Joe Pigott, of RMT, said: 'We should now help the Inland Revenue to plug this loophole, which amounts to an unfair subsidy on foreign crews in the offshore and coastal sectors.'
Tony McGregor, assistant general secretary of the union, has written to the tax authorities urging them to take steps to ensure that tax is being properly collected from non-British seafarers.
He has also sought guidance on what legal obligations are placed on employers to make the necessary deductions and to disclose the tax liability of a workforce.