Unite and the GMB union said jobs at the plant in Nottinghamshire were being filled by Spanish and Polish workers.
But engineering firm Alstom said that over the 30-month lifecycle of the build, British workers would complete the "majority" of the work, and would form the "overwhelming majority" of permanent staff.
A spokesman said: "A project of this type is complex and requires the right mixture of skills. We hire a mixture of British and non-British, EU workers to provide the balance we need.
"We give British firms an equal chance to bid for work on the project. Any claim that there will be no UK workers on the mechanical engineering phase is simply inaccurate and false. UK labour is already being employed in the mechanical phase."
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