Leaders of the Engineering Construction Industry Association believe union officials are behind a series of power-station protests despite the action being officially repudiated by Unite.
The association has also pulled out of the 2010 pay talks because of the ongoing dispute.
Contractjournal.com has seen a copy of the letter sent by association managing director Michael Hockey to leaders of Unite.
It states: "We believe there is evidence that despite formal repudiations by Unite, this unlawful action was supported and organised by some union officials.ADVERTISEMENT
"I now reiterate our requirement that there is an immediate and full and unconditional return to normal working. My members are prepared to take legal action against your union in the High Court if the unlawful action continues.
"Companies concerened have suffered very substantial losses and are considering their remedies both under EU and domestic law to recover these sums.
"Disciplinary action may be also taken against your members. Further, ECIA will not take part in the Naeci Wage and Conditions Review 2010 under such duress."
The letter was sent on February 18. Since then further protests were held at the Staythorpe and Grain power stations on Tuesday
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