Londis has increased the minimum weekly amount its retailers must buy from the group by 50%.
From February 26, the minimum spend for existing store owners will be raised from £2,000 to £3,000.
Londis said the decision, which represents the first change in the company's conditions of membership in more than 10 years, would help improve the strength of the group.
A spokesman explained that it would affect very few retailers as most already spend in excess of the new threshold and that the level remained significantly lower than the group's symbol sector competitors.
Londis retailer David Brunt from Stoke-on-Trent said: "The change in the limit won't affect me as I already spend well above the new level, but I can see where the group is coming from. It should help strengthen the group and stop some retailers from cherry-picking deals."