Britain's competition regulator will include German discounters Aldi and Lidl, and online retailer Amazon, in its probe into Sainsbury's proposed takeover of Asda, as well as long-standing supermarket rivals like Tesco.
"The level and impact of competition presented by newer or growing retailers – including Aldi and Lidl – will be considered," the Competition and Markets Authority said on Tuesday.
Sainsbury's and Asda have both argued that discounters and online grocery should all be included in the CMA's probe.
The inclusion of Aldi and Lidl could be critical in deciding how many stores Sainsbury's and Asda may have to dispose of to get the deal through the regulator.
Analysts at UBS have a base case scenario of a modest 28-54 store disposals to satisfy the CMA. However, that number would have risen to 132-161 stores if the CMA had excluded the discounters in its analysis.
The CMA said it will also look at whether a combined Sainsbury's-Asda could use its increased buyer power to squeeze suppliers and if this could have a potential knock-on effect for shoppers.
Sainsbury's and Asda said in a joint statement they welcomed "the detailed and thorough review" by the CMA.
The CMA expects to issue provisional findings from its investigation early next year, ahead of the statutory deadline for its final decision on March 5.