The Southern Co-operative has recorded a 0.5% drop in like-for-like sales in the two weeks to 4 January.
However, the independent co-op society, which has about 250 stores across 11 counties in the south of England, said total sales rose 5% during the two-week Christmas trading period.
It said its “ongoing investment in the business during 2014” had boosted sales.
“We are pleased to have achieved a growth in sales of 5% at a time of such intense competition in the market, as we continue our expansion plans with 13 new stores opened in 2014 from Kent to Devon,” added Southern Co-op CEO Mark Smith.
Southern Co-op reported a 7.8% increase in annual sales to £327m and an 8.1% increase in EBITDA in May 2014.