Usdaw general secretary John Hannett joined members from M&S outside the store to give out a postcard reminding shoppers that the company makes much of its ethical trading stance, but refuses to recognise Usdaw for collective bargaining.
Union members across the UK will be talking to customers asking them to support their campaign by signing the specially designed postcards.
“Usdaw has seen a large number of hard working M&S staff join Usdaw and they simply want their employer to recognise Usdaw for collective bargaining,” said Hannett.
“Our M&S members are telling us loud and clear that they want an experienced, independent retail union to protect what are still some of the best terms and conditions on the high street.
M&S has estimated that less than 1% of its 66,000 UK employees are members of Usdaw. The company’s staff council - the Business Involvement Groups - has 3,500 elected members.
Usdaw has previously refused to disclose how many M&S staff were union members, only saying it was a “significant number”.