Staff at the Lindsey refinery, which is owned by French energy firm Total, went on strike last week over the employment of Italian and Portuguese sub-contractors on a new construction project.
The dispute triggered sympathy strikes among thousand of workers at other oil and gas refineries across Britain.
Phil Whitehurst, a member of the negotiating committee for the GMB union, welcomed the decision to return to work but said more disputes were likely over the use of foreign labour.
Whitehurst said: "It was an excellent decision. We have now got the chance to go back to work but the fight does not stop here.
"We have got the MPs worried. I think we have got Gordon Brown [the British prime minister] worried. I don't think they know how to deal with us.
"We are not trying to bring the government down, we're just trying to get them to listen."
Total said it was delighted work could resume on the $290m project to expand the refinery.
It said: "We would like to highlight again that we have not, and will not, discriminate against British companies and British workers."
The strikes had not affected energy supplies because they involved workers involved in maintenance and construction.