Asda is to cut up to 300 jobs and reduce the pay of 4,000 staff on the night shift as part of a restructuring to make its operations as "efficient" as possible.
Restocking shifts at 184 stores are to move from night time to the day, which will put 211 managers at risk while 4,137 workers will lose their night pay premium from the shift change.
Working hours are also being reduced at in-store post offices, while seven in-store pharmacies are to shut.
Ken Towle, Asda's retail director, said: "The retail sector is evolving at pace and it is vital we review changing customer preferences, along with our own ways of working, to ensure we are operating as efficiently as possible so that we can continue to invest and grow our business.”
The GMB union suggested the actual number of job losses might be much higher and even hit 4,000.
Asda was acquired by a consortium of the Issa brothers and private equity group TDR Capital two years ago for a debt-financed consideration of £6.8bn.
Reports at the weekend suggested the brothers were mulling a £12bn merger of Asda with their petrol forecourts business, EG Group.