Britain's competition regulator said Sainsbury's $9.5 billion takeover of Walmart-owned Asda should either be blocked entirely or require the sale of a significant number of stores, or even one of the brands.
The Competition and Markets Authority said on Wednesday its current view was "that it is likely to be difficult for the companies to address the concerns it has identified".
In its provisional findings, the CMA said the deal could lead to a worse experience for in-store and online shoppers through higher prices, a poorer shopping experience and reductions in the range and quality of products offered.
It also had concerns that prices could rise at a large number of Sainsbury's and Asda petrol stations.
Sainsbury's, Britain's second biggest supermarket behind Tesco, said it fundamentally disagreed with the findings.
"The CMA has moved the goalposts and its analysis is inconsistent with comparable cases," it said in a statement.
"We will be working to understand the rationale behind these findings and will continue to make our case in the coming weeks."