Balfour’s estate includes 76 c-stores and 35 newsagents based in southern England and had sales of £67m last year.
The Co-op plans to invest £16m in converting the stores to its Welcome format to boost the total number of stores at the group to 1,829.
Malcolm Hepworth, chief operating officer, retail for the Co-operative Group, said: “We needed to continue to build our presence in the South and add to the momentum building behind our convenience proposition.”
Director at 3i, Julian Davison, said the Co-op had “beaten off strong competition to acquire the business at a time when there is a great deal of activity in this sector”.
The Co-op noted that the Alldays estate of 630 c-stores it bought last October for £133m are being converted at the rate of three a week and that refitted stores “are showing sales uplifts of around 30%”.