A number of contract workers at a power station and other sites walked out on an unofficial strike in a dispute over the hiring of labour yesterday.
The wildcat action hit a liquefied natural gas site in Milford Haven, Pembrokeshire, west Wales, and the Aberthaw power station in south Wales and involved laggers and thermal insulation engineers.
It is believed the strikers complained that an agreement by contractors to offer jobs to local labour had been broken.
Sources said the action could spread to other sites across Britain unless the dispute was resolved quickly.
Paul Kenny, general secretary of the GMB union, said: "We understand that an agreement to give work on these large projects to local labour has been breached."
Sources said contract staff at several sites across the UK joined the unofficial walkout, in support of the workers at Milford Haven, where the dispute is believed to have started.
It is understood workers complained that Polish staff were being hired for a contract, in contravention of an industry agreement.
The walkout has echoes of the bitter dispute earlier this year which started at the Lindsey oil refinery near Immingham, north Lincolnshire, over a £200m construction contract being given to an Italian firm.
One shop steward said: "We are having the same problem again and again. When will our politicians realise they have a responsibility to the citizens of this country?"
A spokeswoman for RWE npower, which owns the Aberthaw power station, said: "We understand that some contract workers, who work on either construction or maintenance projects at Aberthaw power station, have left site as part of unofficial action.
"No RWE npower staff are involved and the situation is not affecting operations at Aberthaw power station."
The spokeswoman added that no official reason had yet been given for the walkout.
A spokesman for the South Hook liquefied natural gas terminal at Milford Haven said: "I can confirm that a number of employees of a sub-contractor working on the South Hook site have walked off the site.
"The sub-contractor involved is taking steps to resolve matters with its employees and it would be inappropriate for us to comment further at this time."
A number of contractors also walked out of the ConocoPhillips Humber refinery, in north Lincolnshire.
According to some reports, about 200 contractors walked out in support of the workers in Wales.
In a statement, ConocoPhillips said: "The action has not affected the normal operations at the site and we look forward to the situation being resolved as soon as possible."