Hundreds of workers at the Stanlow Essar oil refinery are to vote over whether to strike in a pay dispute.
Around 350 engineering construction employees, who are members of Unite, are being balloted.
The workers are employed by contractors under the National Agreement for the Engineering Construction Industry (NAECI) and carry out essential repair and maintenance at the oil refinery.
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Unite said its members are "angry" that the value of their pay has been "progressively falling" since the pandemic.
The union said that during Covid, they agreed to a pay freeze "even though they provided essential services throughout". In January 2022, they received a two-year pay deal of 2.5% for 2022 and again for 2023.
Unite added that the Engineering Construction Industry Association (ECIA), which negotiates NAECI with the trade unions, "refused to reopen talks" through 2022.
The ECIA later agreed a non-consolidated supplement in February 2023, which ends in December.
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A two-year deal put forward for 2024 and 2025 averaging 6% per year was rejected by the workers.
Unite general secretary Sharon Graham said: "This offer is completely unacceptable when the oil industry is awash with profits. It does nothing to reverse the shrinking value of these workers’ wages over successive years.
"It also ties these workers into gambling on the economy and inflation in 2024 and 2025 when their finances have already been battered by increasingly unpredictable market forces. Unite stands rock solid with our NAECI members – the ECIA must come back with an acceptable offer."
The ballot for strike action will close in mid-October. If backed, strike action is scheduled to start later that month.
Unite national officer Jason Poulter added: "The anger amongst our membership is such that we are balloting for strike action. The ECIA must acknowledge that without a better offer, falling recruitment and retention for NAECI roles will only get worse.
"Any disruption caused by potential strikes lies squarely at their door – a much improved offer needs to be put forward if this dispute is not to escalate into industrial action."
The ballot comes after over 450 construction workers operating under the National Agreement for the Engineering Construction Industry (NAECI), who work for several different contractors at the site, received a major increase in their bonus payments.
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The agreement came after strike action was due to start on Tuesday, July 18, but was called off after fresh talks were promised.
A spokesman for Essar Oil said: "We are aware of the ongoing dispute between the Engineering Construction Industry Association (ECIA) and Unite regarding the National Agreement for the Engineering Construction Industry (NAECI).
"Essar is not party to these negotiations and Essar employees will not be taking part in this industrial action ballot."