Shopkeeper members of local supermarket chain Nisa are set to vote on the future of the group, as Sainsbury's nears a £130m deal to acquire it.
In a bidding war, Sainsbury's is understood to have been selected by the board as the preferred buyer over its rival the Co-operative Group.
More than 1,300 independent retailers operate 3,000 convenience stores across the country as part of the mutual organisation, some under their own names and others under Nisa branding.
Each member has between one and 250 votes, and Nisa's board would need 50 per cent plus one vote in order to get approval for the sale.
But letters by activist members circulating over the weekend protested that a takeover by Sainsbury's would spell demutualisation for the group, and argued that Co-op would be a better fit. Details of any form of takeover have yet to be ironed out, though some have suggested Sainsbury's could use Nisa as the basis for its proposed franchise scheme.
Despite opposition from some corners, other members sit on the board which reportedly approved the Sainsbury's bid. The full range of member opinions may only become known when a vote, for which no date has been set, takes place.
Nisa became a target for acquisition following Tesco's buyout of Booker for £3.7bn, as the big players in the groceries sector bulk up through acquisitions. Sainsbury's already acquired Argos and Habitat last ear for £1.4bn.
The landscape changed again at the end of last week, when it was revealed that Amazon had spent $13.7bn (£10.7bn) on organic food chain Whole Foods, sending shares falling in Tesco and Sainsbury's by as much as four per cent.
Nisa has undergone a turnaround process in the last two years, which saw it swing from a loss in 2015 to underlying profit of £7.3m in 2016, and forecasts indicating £8.5m for 2017.
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Nisa and Sainsbury's both declined to comment.