The GMB union claimed retail giant Amazon had classed staff taking part in its first ever UK strike this week as 'a no show - effectively an unauthorised absence'.
Hundreds of Amazon workers in Coventry have walked out in a pay dispute.
The union said staff had been offered a pay rise of just 50p an hour.
However, workers are now worried they will be fined or even face a gross misconduct charge due to the way their absence has been categorised.
The GMB said mandated industrial action gave workers the legal right to withdraw their labour, which should not be classed as unauthorised absence.
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Stuart Richards, GMB senior organiser, said: “We hope and believe this is just an error on Amazon’s part, rather than an attempt to intimidate workers taking legal industrial action.
“But Amazon needs to sort it out and quick. Coventry workers did an incredibly brave thing taking on one of the world’s biggest companies.”
Rosa Curling, director at solicitors Foxglove, said the move by Amazon was “shocking”.
She said: “If these reports are true, Amazon is acting unlawfully. Amazon needs to make this right and quick.”
Amazon was approached for comment, as well as asked to respond to claims made by its employees about “severe” conditions, in which they were constantly monitored.
It had not responded as we went to press.