More than 1000 M&S staff have been contacted by shopworker’s union Usdaw, urging them to join.
The news follows an ongoing pay row between management and staff over changes to pay and pensions.
The union has held over 11 meetings with staff in places like Manchester, Liverpool and Stockport, reaching out across the UK to court membership.
M&S’ new contracts for staff were proposed earlier this month. The contracts have been accused by MPs as an attempt to sidestep paying the National Living Wage which is set to continue to rise until 2020.
Pension contributions would be reduced and double pay on Sunday’s and bank holidays would be cut under the new proposed contracts.
Usdaw reports they have seen a rise in membership since the pay cuts were proposed and the meetings with employees can see staff attend in their hundreds.
Mike Aylward, an officer for Usdaw, said many employees were too frightened to fight the proposed contracts but if enough staff joined the union, they would meet with M&S management to become officially recognised as representatives.
Currently staff are represented by National Business Involvement Group (National BIG), but Usdaw said they were ineffective and effectively being used by M&S staff.
"The proposals include one of highest pay rates and one of the best benefits packages in UK retail," an M&S spokesperson said.
"The consultation is ongoing and we are listening carefully to our colleagues and their representatives."