Unite protesters will target more than half a dozen William Hill shops tomorrow over restructuring plans the union has dubbed an "attack" on workers' pay.
Protests will be staged outside branches in nine locations including central London, Liverpool, Manchester, Leeds and Southampton on plans Unite has said will slash shop managers' pay by an average of £6,000 a year.
William Hill is currently consulting on proposals to replace the roles of shop manager and deputy manager with a new "customer experience manager".
"Hundreds of loyal and hardworking shop managers and deputy managers face losing thousands of pounds in pay under plans which will see their wages shrink by about £6,000 on average to between £16,662 to £20,721 a year," Unite national officer Rhys McCarthy said.
"It's very poor odds for our members, who are being asked to do the same job for less pay, and a far cry from the sizeable pay off former top boss, James Henderson, is likely to walk away with for his two short years at the helm."
However, William Hill has disputed Unite's interpretation of the shake-up to its retail arm, claiming it has used "inaccurate figures".
The high street bookie has said the proposals to modernise its retail business will provide increased management support by creating 359 jobs in a business performance manager role. New field managers will oversee an average of seven shops compared to the 25 currently overseen by district managers.
"The changes will simplify and bring greater clarity to the shop structure by removing a number of historical grades and anomalies and streamlining to just two roles with no reduction to headcount," the company said in a statement, adding that nine out of 10 shop-based colleagues will see their pay increased.
A William Hill spokesman told City A.M.: "Unite is using inaccurate figures – the fact is many colleagues will benefit from these changes financially, many are being promoted and for those that are negatively impacted we are protecting their salaries for a minimum of 12 months and are consulting with this smaller group to mitigate the impact.
"After these changes are implemented there will be better career prospects for our teams and rewards that are geared to individual performance."
Earlier this year, William Hill rejected a three-way takeover offer from online rivals Rank Group and 888 Holdings.
Although it is one of the most recognised names on the high street, it has struggled to stay ahead in an industry moving increasingly online and issued a £25m profit warning in March due to a drop in its online business.