Trade union Mandate have warned Tesco that they will ballot their 14,000 strong workforce on industrial action if they persist with plans to cuts to workers' pay.
The company plans to move almost 1,000 long-serving staff to new contracts from mid-April.
Tesco say they are engaging in talks with the unions.
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Assistant general secretary of Mandate Gerry Light said that a strike is a real possibility.
"It's becoming more real and I think it's become more serious," he said.
"I think we've a couple of barriers to cross before we get there.
"Again, it's an indication, I think, of the changing face of industrial relations within Tesco, I think it's becoming more adversarial the way they're approaching industrial relations.
"I think [on] the whole notion of a strike threat, the company now have to make a decision. And it's entirely their decision."