Nisa member Anglian Convenience Stores has sold out to the East of England Co-op, blaming market conditions for forcing the sale.
The deal covers 20 c-stores in Norfolk. Anglian managing director Bob Surridge told C-Store: "For the past two years we have been unable to buy any new stores, so we have been under increasing pressure.
"Costs are going through the roof. We tried to buy stores to offset this but were gazumped time and again. We were offering big money, but we simply couldn't afford to pay the prices that the big boys could.
"You can't stop Tesco and Sainsbury's moving into the high street, but there should be legislation to stop them saturating local areas. It can't be right for one chain to have six or eight stores in the same town."
Bob revealed that the chain had received offers before, including those from the big multiples, but they wanted to cherry-pick.
"We knew that if we didn't sell as a chain we would have been forced to sell off the good stores one by one, and it would have been a slow death.
"I am pleased that East of England has bought us - it is a sizeable group and our staff will have a good opportunity to progress with them."
The acquisition brings the number of food stores owned by East of England to 137. All the outlets will be rebranded as Co-op food stores. Bob and his business partner Pete Head will stay on for six months to help with the transition.
Nisa-Today's chief executive officer Neil Turton commented: "It's a great shame that Anglian Convenience Stores has sold. We wish Peter and Bob the very best for the future."
Anglian is the latest of several large Nisa members to sell their businesses in the past two years. Turton pointed out that while the latest exit was disappointing, the group has recruited members with a combined turnover of more than £147m in the financial year to date.