Approximately 300 contract workers walked out at ConocoPhillips' (COP) 221,000-barrel-a-day Humber refinery in the U.K. Monday, the company said.

"We have lost a small percentage of our contract workforce," the company said in an e-mailed statement. Around 300 of the 1,200 contractors employed at the site participated in unofficial strikes Monday.

More than 2,000 contract workers at energy facilities across the U.K. halted work Friday to join a strike over sweeping job cuts at an oil refinery owned by French oil major Total SA (TOT). Total said it plans to lay off 647 contract employees Friday at its Lindsey refinery in Lincolnshire, eastern England, angering unions and workers. The Unite union put the total job losses closer to 900.

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

(Angela Henshall in London contributed to this report.)