The Co-operative Group is to extend its store opening hours next month, provoking outrage among staff.

The society is understood to have emailed stores last week advising of the changes and to find out whether they would have staff available for the extra hours. Typical store opening times are currently 7am to 10pm, but stores have been asked to open from 6am to 11pm, and from 5am until midnight in some cases.

Staff flooded Facebook to rage about the plans, which The Co-op said was to meet customer demand. “Hardly see anyone after 10pm,” said one member of staff. “Definitely not enough to stay open. We have an 11pm licence but that doesn’t help.”

Concerns were also expressed over safety, with staff worried about leaving the store later. “Can you imagine how safe everyone will feel finishing at 11,” said one member of staff.

Others remained positive. “Mine is open until 11pm. I do it every Sunday, nobody ever really comes in much after 10pm but it’s a great chance to get faced up.”

The extended hours would put the society’s more on a par with those of rivals Tesco Express and Sainsbury’s Local and comes as Kantar Worldpanel figures this week revealed sales in The Co-op stores fell 1.3% in the 12 weeks to 19 February. Market share also slipped from 6.7% to 6.3% year-on-year.

A leaked memo this week also claimed like-for-like sales in former Somerfield stores now owned by The Co-op were down 3.38% in the eight weeks to 25 February.

The Co-op would not give further details, but last week said: “The Co-operative was continuing to invest heavily within the food business and this included a planned increase in opening hours across its retail estate to meet customer demand.”