The union won a successful case for the laggers after they were found to be underpaid and having unfair deductions from their wages while working at the Uskmouth Power Station.
The award was made against German based thermal insulation subcontractor Damar earlier this year.
But the GMB said the firm has quit the UK market without paying the money.
Kathleen Walker Shaw GMB European officer said: “This case has exposed a hole in EU posting of workers Directive which must be closed.
“GMB will not let the matter rest and will continue to seek support for changes to the Directive regulations to close this legal hole.
“There can never be a single labour market in the EU if companies are able to simply walk out of an EU jurisdiction leaving workers high and dry”.