Staff have raised concerns about the retailer's proposal to introduce a shift that runs from 10pm on Christmas Eve to 7am on Christmas Day.
"Previously we have moved the Boxing Day bank holiday to Christmas Eve, which enabled the crew to have the night off and have Boxing Day as a normal working night. Now they want Christmas Eve as a normal night and Boxing Day night as bank holiday," said an insider.
A superstore manager said a decision on the working hours would be made later this year. "Some colleagues do not celebrate Christmas, so I think they would be happy to work in return for time off during other celebrations, such as Eid," he said.
Some staff already had to finish later than normal last Christmas Eve compared with previous years because the retailer opened more stores on Boxing Day, he added. "My store opened on Boxing Day for the first time. It was really successful, but we had to work until about 9.30pm on Christmas Eve."
The retailer opened 257 larger stores on Boxing Day 68 more than in 2008.
A spokesman would not disclose the retailer's Christmas plans.