"This is of course our busiest time of year but it was important for us to give as many of you as possible the opportunity to spend this time with those loved ones that you may not have not seen for many months so, uniquely for this year, we will not reopen our stores until 27 December."
Roger Jenkins, national officer of the GMB union, said: "It's a shame this is not extra holiday - workers will have to book a day of their annual leave entitlement.
"But it's a step in the right direction and GMB now calls on the rest of the retail sector to follow suit and repay these key workers with a chance to spend Boxing Day with their loved ones."