Workers hired to build the flagship £2.6bn Beatrice offshore windfarm in Scotland have included migrants without proper immigration documents paid a fraction of the UK minimum wage, the Guardian can reveal.

Offshore windfarming is one of the UK’s biggest growth industries, hailed by both the Conservatives and Labour as a priority for investment that will create thousands of jobs while also producing clean energy.

The International Transport Workers’ Federation (ITF) and the Rail, Maritime and Transport Workers’ union (RMT) have said, however, that the use of cheap foreign labour instead of local workers is a growing problem in the sector’s subcontracting chains. They have accused the government of failing to protect workers.

A group of Russian workers recruited as relief crew on the giant crane ship contracted to carry out the initial construction on the Beatrice windfarm were detained by immigration officials at Aberdeen airport in April 2017.

They were being brought in to the country on seafarer identity documents, intended for foreign crew leaving UK waters immediately, instead of the official permits required of people from outside the European Economic Area (EEA) entering the UK to work.

Permits have not historically been available for lower skilled people from non-EEA countries work on vessels that operate in the UK’s territorial waters.

But in an extraordinary relaxation of immigration rules, the Home Office granted a six-month, time-limited waiver to the windfarm industry to use non-EEA workers on 21 April 2017. The ITF said it was needed to give the Russians stuck at Aberdeen airport leave to enter.

An ITF inspection on board the crane ship in the Moray Firth the following month found more than 140 migrant workers, the majority from Russia and some from Indonesia.

Contracts for some of them, seen by the Guardian, set rates of pay that were a fraction of the UK minimum wage.

In the immigration concession the Home Office said: “Firms involved in the construction and maintenance of windfarms within territorial waters should look to regularise the position of their workers.” However the concession has since been extended twice and is now valid until April 2019.

Amber Rudd, who was home secretary at the time of the concession, had been energy secretary when the Beatrice project was launched, and promised it would “provide home-grown clean energy boosting skills and create jobs and financial security for working people and their families in Scotland and across the UK”.

Beatrice is a joint venture led by the privatised utility Scottish and Southern Electricity (SSE), and is one of the largest investments made in Scottish infrastructure. It is due to be completed next year.

The SSE consortium won a licence to develop the windfarm in UK territorial waters with pledges that it would aim to recruit locally. It subcontracted part of the construction to a leading firm specialised in offshore operations, Seaway Heavy Lifting (SHL).

SHL deployed one of its giant crane ships, the Stanislav Yudin, to work on the project a few miles from the Scottish coast.

Contracts issued by SHL and an agent to Russian workers arriving to join the Stanislav Yudin in 2017 were for 12-hour days, seven days a week while at sea – at just £58 to £68 per day. Some workers were receiving less than £5 an hour when the national minimum wage was £7.50.

SSE and its subcontractor say migrant worker contracts were altered when they became aware of irregularities, but according to the unions, Beatrice exposes a lack of adequate enforcement and regulation in the booming sector.

SHL said the concession was industry-wide and it was committed to carrying out its business in an ethical manner. “We have been working closely with the Home Office and relevant industry bodies regarding concerns raised around the employment of non-EEA nationals in UK territorial waters. We continue to work to ensure we meet all applicable regulations,” it said.

An SSE spokeswoman said: “SSE, and its project partners, take any potential breach of UK immigration law by its contractors extremely seriously and each of [Beatrice’s] suppliers is contractually required to fully comply with UK law, including obtaining all relevant work permits and paying at least the national minimum wage.

“SSE, along with its [Beatrice] partners, took action as soon as the issue came to light to ensure SHL was meeting its contractual and legal obligations.”

The government has provided generous subsidises to Beatrice and other windfarms, and jobs in the sector are seen as particularly important to Scotland and the north-east.

The Guardian has identified several other low-paid migrant workers being used on guard vessels for other windfarm construction and offshore cable-laying sites.

Local fishing trawlers often take on contracts for guard duty as a lucrative way to supplement their income when they have run out of fishing quota, and many depend on it for survival. Several workers brought in from Ghana are doing these jobs, at a reported rate of £1,050 per month, with hourly pay well below the UK national minimum wage.

The ITF maritime coordinator, Jacqueline Smith, said it had become aware of a sharp increase in the number of Filipino and Ghanaians on board trawlers in recent months, which it suspects relates to the increase in offshore wind farm activity.

The RMT general secretary, Mick Cash, called on the government to enforce the law and prevent the offshore industry importing cheap labour.

The Department for Business, Energy andIndustrial Strategy said any workers under the concession not receiving their entitlement to the national minimum wage could contact the conciliation service Acas, and that HMRC was committed to cracking down on employers who break the law.

HMRC declined to say whether it was investigating any breaches.

The Home Office said it “agreed to grant a concession as a temporary arrangement, outside of the immigration rules, to workers deemed to be essential to the construction and maintenance of wind farms within territorial waters.

“This temporary arrangement was put in place to give the industry the opportunity to regularise its arrangements for their employees. The continued need for the concession is kept under review.”