The site employs 550 staff and has up to 1,000 contract workers at any time
Hundreds of workers took unofficial industrial action on Thursday in a fresh row over jobs at an oil refinery in North Lincolnshire.
A GMB union official said the workers had walked out in a dispute about job losses at Lindsey Oil Refinery.
He said there were concerns staff were being "victimised" for taking part in other walkouts earlier this year.
The refinery's owner Total confirmed workers were on strike but said the action was not affecting the plant.
It is understood that a sub-contractor at the refinery announced 51 job losses, while another employer on the site was hiring staff.
The staff who walked out were contract workers on a new unit at the site, which has been at the centre of a bitter row over the recruitment of non-UK workers over the past few months.
The refinery employs 550 staff and has up to 1,000 contract workers at any time.
In a statement, Total said the workers walked out early on Thursday morning and demonstrated outside the refinery.
Protests were held in February over the recruitment of foreign workers
It said: "The rest of the refinery is continuing to operate normally.
"Total is working with the contractors and the union to resolve this situation as soon as possible."
Paul Kenny, general secretary of the GMB union, said: "GMB have not been contacted by the employer about any dispute.
"However, we understand that there seems to be a dispute about compulsory redundancies on the site while other workers are being recruited into similar type jobs.
"There are concerns that these workers may be being victimised for earlier industrial action and that these redundancies are in breach of the agreement that brought industrial peace to the site some months ago.
"We understand that local discussions are under way to try to resolve the dispute."