Supermarket giant Asda has started consulting with workers in 18 of its stores over changes to staffing because of different shopping habits such as ordering online or self-scanning products.
Staffing arrangements in a further 59 stores are also being looked at.
No store closures are being planned and there are no figures for possible job losses.
An Asda spokesman: "We are currently in discussions with a number of our stores about changes that are needed to the number of hours required to run that store for our customers.
"It is common practice for a supermarket to need to make changes to hours based on the changing shopping habits of customers, and as ever we'll work with our colleagues to try and find the right way to ensure we have the right people, in the right place at the right time for our customers.
"We understand that any conversations about change are unsettling but it is always our upmost priority to find alternative roles or working patterns for impacted colleagues."
Gary Carter, national officer of the GMB union, said: "GMB is in consultation with Asda over the situation and is currently in discussion with senior management."
Asda employs 165,000 workers in 645 stores.