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The strikes started after 50 workers at the South Hook liquid gas plant in Milford Haven, west Wales, complained that an agreement to employ local labour had been breached.
The workers claimed the contractor Hertel UK hired about 50 Polish workers to fill vacancies at the plant.
Following news of the walk-out in Wales, 200 staff at the Fiddlers Ferry power station in Widnes, Cheshire, also stopped working in sympathy, and another 200 workers walked out at the ConocoPhillips Humber refinery in north Lincolnshire.
Workers at Aberthaw power station in south Wales and the Dragon LNG site also walked out.
The demonstrations are reminiscent of strikes held at the Lindsey oil refinery in Lincolnshire in January, which were sparked after Italian and Portuguese workers were given all the jobs on a new contract.
Hertel UK has said it tried to source local labour but could not find the necessary skills.
The contractor's managing director, David Fitzsimons, told the Daily Mail: "For this particular project we have looked to employ local labour. However, despite our efforts we were unable to source skilled people for the start of the project.
"As a result, we subcontracted the works to a third party, which also employs non-UK labour. This is a short-term project and our involvement will be for approximately three months. We have agreed with union officials that we will hire local skilled personnel once they are identified."
However, Paul Kenny, general secretary of GMB union, said Hertel UK had "deliberately flouted" the agreement.
He said: "I understand that a contractor, Hertel UK, has point blank refused to adhere to this written agreement.
"This deliberate flouting of an agreement has angered members employed on this site and I am informed that they have walked out. There is widespread anger and outrage at repeated attempts in different projects around the country to exclude local people from job opportunities on these projects.
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