The Sita Sembcorp consortium is developing the plant which has been the focus of long-term union protests.

A Sita spokesman confirmed that the consortium is looking into union allegations following a site visit by officials last week.

Main contractor on the job is the CNIM Clugston consortium who started on the 33 month build programme in January 2014.

Unite, GMB and Ucatt visited the job with independent interpreters to talk with migrant workers.

The unions claim some workers were “being paid barely above the minimum wage and having to pay employee and employer national insurance contributions because they were employed through umbrella companies.”

Unite regional officer Steve Cason said: “We have suspected all along that the undercutting of pay and the exploitative use of umbrella companies is taking place on the site.

“Now we know it’s true and that a coach and horses is being ridden through national agreements established to maintain standards and ensure workers are paid a fair day’s pay for a fair day’s work.”

A Sita Sembcorp statement said: “SSUK has subsequently written to CNIM Clugston demanding a detailed response to these issues and, more importantly, evidence to either substantiate or refute each individual claim.

“A time frame of seven working days to respond was committed to with the unions, in order to allow both SSUK and CNIM Clugston to investigate these matters properly. That time frame has not yet lapsed.

“SSUK is placing substantial pressure on CNIM Clugston to respond to these allegations and we will respond to the unions, and the wider project stakeholders, in due course and with a factual basis.”