Asda discriminated against trade union members, an employment tribunal has ruled.
Workers at a depot in Washington, Tyne and Wear, had voted against a move to bring their conditions in line with non-union workers at a similar depot.
The Wal-Mart owned retailer was found to be in breach of trade union legislation and now faces paying out £850,000 in total - up to £2,500 to each of the 340 workers.
Asda said it was disappointed with the ruling and is considering an appeal.
Paul Kenny, acting general secretary of GMB, said: "Last year Asda offered GMB members in Washington a pay rise of 10 per cent if they would give up their membership, but our members rejected this.
"Asda have been found guilty of trying to bribe their way to a union free company. They have now been directed to pay £850,000," he told the BBC
An Asda spokesman said: “When we bought the depot from Wincanton a number of years ago we protected the workers' existing terms and conditions.
"After many requests for them to be brought into line with an Asda owned and operated depot close by we gave them the opportunity by ballot whether they supported the move.
"They voted against any changes and were kept on the existing terms."
