The refinery near Immingham is the third largest in the country
Workers at a huge oil refinery have walked off the site, claiming bosses have brought in 90 foreign workers.
Some 300 staff took part in the unofficial action at Lindsey refinery near Immingham on Wednesday and a picket is planned, a union leader said.
Unite shop steward Garry Scales said: "We are angry that workers have been taken on from outside the UK when people here are out of work."
Owners Total said operations on the 500-acre site were not affected.
Mr Scales said management had claimed the staff taken on were in specialist trades but he alleged they were, in fact, doing a variety of work which could be done by UK workers, such as welding and pipefitting.
Several workers told BBC Radio Humberside they had been advised by union officials against taking immediate action but had gone ahead and stopped work.
In a statement, Total said: "A number of contractors working on a project at Lindsey oil refinery have staged a walkout this morning.
"The unit is separate to the main refinery, and as such, this action has not affected the normal operation of the refinery.
"The main contractor, Jacobs is currently having discussions with the workers and union representatives. We hope the situation will be resolved as soon as possible."
The site on Eastfield Road is the third largest refinery in the country, processing more than 200,000 barrels a day.
It opened 40 years ago to process oil from North Sea wells.