The Co-op has admitted to 107 breaches of competition rules by unlawfully blocking rival supermarkets from opening near its stores.
The Competition and Markets Authority (CMA) found the convenience store chain had violated an order limiting supermarkets' ability to prevent nearby land being used by competitors.
This practice restricts competition and reduces consumer choice, potentially preventing shoppers from accessing cheaper prices.
The CMA has expressed concern that these breaches demonstrate "a significant failure of compliance" for a business of Co-op's size.
The CMA has expressed concern that these breaches demonstrate 'a significant failure of compliance' for a business of Co-op's size
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The watchdog said it was "ensuring that shoppers have more choice and so benefit from a wider range of groceries and access to cheaper prices".
Co-op owns almost 2,400 stores across the UK and holds a 5.2 per cent market share in the £190.9billion supermarket industry.
The breaches relate to the Groceries Market Investigation (Controlled Land) Order 2010, which prohibits supermarkets from imposing restrictions that block rivals - which has been in place for almost 15 years.
Co-op has re-written 104 of the problematic agreements and has committed to resolving the remaining three.
These breaches involves restrictive covenants that prohibit land being used for a supermarket.
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The order also bans exclusivity arrangements preventing landlords from allowing stores to compete with an existing supermarket for periods longer than five years.
The CMA noted that Co-op has "made a considerable effort to amend all their unlawful agreements" despite the order being in place since 2010.
Senior director of markets at the CMA Daniel Turnbull said: "Restrictive agreements by our leading retailers affect competition between supermarkets and impact shoppers trying to get the best deals.
"We know that Co-op has made a considerable effort to amend all their unlawful agreements, given this Order has been in place since 2010."
Co-op's 107 breaches far exceed those of other major supermarkets that have faced similar CMA action in recent years - although Morrisons previously held the highest number at 55 breaches
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He added: "Co-op and the other designated retailers must make sure they do the right thing by their customers in the future."
A spokesman for Co-op said: "As a business that is committed to operating fairly, we recognise this is extremely disappointing.
"Co-op operates in a range of markets, both as a community retailer and a national funeral provider and the number of breaches amount to less than two per cent of transactions across our entire property portfolio.
"This is a matter we take very seriously, and we have taken all necessary action to ensure this issue is resolved and does not happen again."
Co-op's 107 breaches far exceed those of other major supermarkets that have faced similar CMA action in recent years - although Morrisons previously held the highest number at 55 breaches.
Tesco was found to have 23 infractions in 2020, while Waitrose had seven in 2022 and, last year, Sainsbury's had 18 breaches, Asda 14, and M&S 10.
