The GMB union is threatening to sue recruitment agencies that provide temporary staff to the supermarket giant Asda.
The union has written to agencies warning them that it is illegal to provide cover during a strike at 20 of Asda's distribution depots and says it will prosecute if agencies fail to heed its warning.
Paul Kenny, GMB general secretary, commented: "Ignorance of the law could be used as an excuse, so we have written to agencies to make the position clear. If they supply staff anyway, then we will be taking legal action."
GMB members working in Asda Wal-Mart's 20 distribution depots voted three to one to strike. Members are striking for the establishment of national bargaining structures between its members and Asda, covering pay, conditions and union facilities.
The union will post six members at the gates of each depot equipped with surveillance devices to ensure the law is not broken.
"We're looking to record everything that happens on the picket line, so if anything untoward happens we can prosecute," said Kenny.
Legal officer Barry Smith said the union has used firms including Thompsons, Rowley Ashworth and Pattinson Brewer in the past, but would "take stock as things develop" in the present conflict.
However, Asda spokesman Dominic Burch described the letter to agencies as "a waste of a stamp". "We have no intention of breaking laws or doing anything illegal," he said. "It's a red herring by the GMB."