Around 50 workers held a demonstration at Aberthaw in January
An unofficial strike over the hiring of foreign contract workers at a liquefied natural gas (LNG) terminal has spread to other plants.
The walk out started with 50 laggers at South Hook LNG in Milford Haven, Pembrokeshire. The contractor said it had tried to find local skilled labour.
It comes only a week after the terminal was officially opened by the Queen.
Workers also walked out in sympathy at plants as far afield as Vale of Glamorgan, Lincolnshire and Widnes.
Hertel UK managing director David Fitzsimons, said the company employed more than 3,500 UK personnel, including 150 apprentices and trainees.
"For this particular project we have looked to employ local labour," he said.
"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 and was also engaged on phase 1 of this project in 2008."
"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.
"I regret that this situation has escalated. We understand people's concerns and we have engaged in detailed discussions with trade union national officials and local representatives."
Hertel and its sub-contractors said they complied with national frameworks on terms and conditions for workers.
Earlier, Paul Kenny, general secretary of the GMB, said his union was making urgent inquiries into the cause of the dispute.
He said: "We understand that an agreement to give work on these large projects to local labour has been breached."
Mr Kenny added: "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. The political spotlight needs to be turned on these companies so that discriminatory employment practices cease immediately.
"People are at end of their tether and patience with the clients who engage these subcontractors."
South Hook was officially opened as a LNG terminal by the Queen earlier this month.
A spokesperson for South Hook confirmed a number of employees of a sub-contractor had 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," said the spokesperson.
The dispute soon caused sympathy action at other plants.
Laggers and thermal insulation engineers walked out at Aberthaw power station in the Vale of Glamorgan.
A spokesperson for RWE Npower says some contractors have walked off site at Aberthaw power station but they have not had any formal notification about their reasons.
All RWE Npower employers are at work and the site is operating.
Some workers also walked out at Dragon LNG site, a few miles away from the South Hook site in Milford Haven.
A spokeswoman for Dragon LNG, which is currently building a smaller terminal the other side of Milford Haven, said some workers had left the site in support of industrial action elsewhere, but it was not a mass exodus and there was no demonstration.
Meanwhile, around 400 workers in Widnes and north Lincolnshire walked out in sympathy.
It is also reported that "a handful" of subcontractors at the Fiddlers Ferry power station in Widnes stopped work although they are expected to return to work on Wednesday.
A number of contractors also walked out of the ConocoPhillips Humber refinery in North Lincolnshire.
This site at South Killingholme is next to the Total Lindsey Refinery at the centre of the action earlier this year.
According to some reports, about 200 contractors walked out in support of the workers in west Wales.
In a statement, ConocoPhillips said: "This morning a number of contract maintenance workforce walked out at the ConocoPhillips Humber Refinery.
"The action is unrelated to ConocoPhillips' local employment practices and no ConocoPhillips staff are involved. The action has not affected the normal operations at the site and we look forward to the situation being resolved as soon as possible."