Hundreds of contract workers at U.K. energy facilities held a second day of unofficial strikes Wednesday to protest the use of foreign labor.

The latest wave of walkouts - which echoed similar industrial action in January and February - was triggered Tuesday by contractors at the South Hook liquefied natural gas terminal at Milford Haven in Wales.

The workers said Netherlands-based industrial services company Hertel had been unfair to local workers in its hiring for an insulation project, according to the U.K.'s GMB Union.

The South Hook contractors resumed their strike Wednesday, GMB said. Negotiations took place late Tuesday between union representatives and Hertel, and were expected to continue later Wednesday, the union added.

"A subcontractor working on the South Hook site is involved in ongoing discussions to resolve the situation," a spokesman for the South Hook LNG Terminal Company Ltd. said.

Workers at ConocoPhillips' (COP) 221,000 barrel a day Humber refinery in northern England walked out for a second day in sympathy with the South Hook workers. They were joined by contractors from Total SA's (TOT) nearby Lindsey oil refinery. Operations weren't affected at either refinery.

Around 800 protestors participated in a peaceful protest near the refineries, local police said.

Meanwhile, 300 contract workers at RWE AG's (RWE.XE) Aberthaw Power Station in Wales downed tools for a second day and 100 contractors at U.K. power company Scottish and Southern Energy's (SSE.LN) Fiddlers Ferry power station walked out. Fifty contractors employed by Hertel at Chevron Corp.'s (CVX) 220,000-barrel-a-day Pembroke refinery also left their posts.

In a statement Tuesday, Hertel said it had sought, but was unable to secure, skilled local workers for the three-month insulation project. It subcontracted the works to a third party that employs non-U.K. workers.

"We have agreed with union officials that we will hire local skilled personnel once they are identified," Hertel managing director David Fitzsimons said in an emailed statement. "We understand people's concerns and we have engaged in detailed discussions with trade union national officials and local representatives."

Hertel said it complied with national labor agreements on terms and conditions for workers.

In January and February, hundreds of workers walked out of U.K. refineries, power plants and other energy facilities after Total awarded a construction subcontract at the Lindsey oil refinery to an Italy-based company Irem SpA.

-By Lananh Nguyen, Dow Jones Newswires; +44 (0)20-7842-9479; lananh.nguyen@dowjones.com

(Angela Henshall, Reza Amanat, Selina Williams and Nick Heath in London contributed to this report.)