The DTI's cost-benefit analysis into easing or removing restrictions on Sunday trading in England and Wales has "serious shortcomings", according to the Association of Convenience Stores.
The DTI's report by Indepen said relaxed trading hours would be worth as much as £20.3bn to the economy over the next 20 years.
However, the ACS said that it had commissioned an appraisal of the analysis that cast doubt on its claims.
The report assumed there was majority consumer support for extended hours, that large stores alone supplied fresh produce on a Sunday and online shopping offset the possible loss of convenience shopping for a majority of households.
Spar warned that the profit margins of small stores could be slashed by almost a third if current Sunday trading laws in England and Wales were relaxed.