Parc Adfer plant recruitment attacked by GMB and Unison
An £800m energy project backed by public funding has been accused of "exploiting" foreign workers and failing to benefit the Flintshire community by two trades unions.
GMB and Unite have claimed the Parc Adfer incinerator on Deeside is not employing local people.
They want the energy-from-waste scheme to adopt an industry agreement.
But the main contractor has rejected the comments and said the agreement was optional.
Once operational in 2019, Parc Adfer will generate renewable electricity for more than 30,000 homes - enough power for a town nearly four times the size of nearby Connah's Quay.
The unions want the project to adopt the National Agreement for the Engineering Construction Industry (NAECI) which would mean higher wages and improved terms and conditions.
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They also believe the benefits of the investment will not circulate around the north Wales region.
But main contractor CNIM said the project was not designated as a NAECI and it would not become one now.
The GMB accepted that the 17 megawatts (MW) facility falls below the 50MW that would make NAECI guidelines mandatory.
But the unions argued that it is a "missed opportunity" - particularly as the £800m is publicly funded by five Welsh councils: Flintshire, Anglesey, Gwynedd, Conwy and Denbighshire.
They believe the investment would create more jobs and benefit local businesses if the councils had insisted on NAECI rules as part of the planning process that gave the project the go ahead.
The GMB accepted the use of this national agreement would increase costs by 60%.
Wages at the site could be 63.5% lower than under NAECI guidelines.
CNIM said the company "has been clear that it complies fully with UK law and observes all relevant UK legislation".
"This project has been procured by the local authorities, who are duty bound to make the best use of public funds.
"The UK government is clear that NAECI is not specified as a requirement for local authority projects. As a result, the project is not designated as being a NAECI site and will not become one.
"There are no local authority projects to date that have been designated NAECI and as far as we are aware no project has become one after procurement."
As well as worries over how much the north Wales economy will benefit from the project, the unions are concerned about health and safety at the site.
They believe workers, who do not have the skills required for the jobs undertaken, are being employed on the national living wage or the national minimum wage, and under zero hours contracts.
Under the NAECI framework, unions would have the authority to audit health and safety as well as pay and conditions at construction sites.
The GMB is calling for the Welsh Government to intervene based on its ethical procurement code of practice.
This commits public, private and third sector organisations to a set of actions "that tackle illegal and unfair employment practices" according to the Welsh Government's website.
Commenting on the code, a Welsh Government official said: "All organisations that receive funding from Welsh Government, either directly or via grants or contracts, will be expected to sign up to the code.
"Other organisations in Wales are encouraged to sign up.
"We are committed to ensuring the highest standards of employment practice exist in Wales, in line with the standards in the code."
The unions said they will step up their campaign in January with protests and demonstrations planned.
The plant is due to operate for 25 years and create an estimated 35 full-time jobs.