Workers began a work to rule at Grangemouth oil refinery in Falkirk on Monday of this week.
They oppose Ineos bosses’ attempts to victimise Unite union rep Stephen Deans.
The action includes a continuous overtime ban, work to rule and withdrawal from secondments and special projects.
It will slow down operations at the refinery—the only one in Scotland—and Unite has said members are losing patience and could escalate.
A strike would hit fuel supplies across much of Britain.
Ineos has suspended Stephen, allegedly in relation to his role in Falkirk Constituency Labour Party.
Stephen has already been cleared of wrongdoing by the Labour Party and the police.