Sir Vince Cable, leader of the Liberal Democrats and the former business secretary, said the CMA "must investigate" any deal after it emerged the companies were in advanced talks.
Sainsbury's, Britain's No. 2 supermarket, and Asda, the United Kingdom arm of Walmart that is the No. 3 player, are close to securing a merger deal, a source with knowledge of the situation told Reuters on Saturday.
It is understood that any investigation by the CMA would only be announced once an intention to merge was formally unveiled.
Rebecca Long-Bailey, shadow business secretary, echoed Sir Vince's calls, warning the merger risks "squeezing what little competition there is in the groceries market even further".
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One source close to Wal-Mart said the merger would allow the two companies to invest in providing more products at lower prices.
The combined group would comprise 2,800 stores and would represent 31.4% of the United Kingdom grocery market - slightly more than that of market leader Tesco.
News of the potential deal also sparked concern among workers' unions, which demanded urgent meetings with Sainsbury's and Asda chiefs.
In a statement on Saturday (28th April), the company said: "J Sainsbury plc ("Sainsbury's") notes the speculation concerning a possible combination with Asda Group Limited ("Asda")".
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Joanne McGuinness, Usdaw national officer, said: "Our priorities will be to protect our members and ensure any deal between the retailers does not impact on their jobs or incomes".
In recent years, both businesses have seen their market share squeezed by stiff competition from budget outlets Aldi and Lidl, however, the proposed move would see Britain's second and third largest supermarkets merge to become the Brothers of Destruction of the retail world.
Britain's traditional retail industry is under enormous pressure from the prolific growth of online rivals which are not saddled by the fixed costs of bricks and mortar players.
In 2016 Sainsbury's paid £1.4 billion to take over Argos and Habitat owner Home Retail Group and has been expanding its ranges and presence since.
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The deal comes at a time of major upheaval in the United Kingdom retail sector as Aldi and Lidl become increasingly powerful and the cost of running high street stores increases because of higher business rates and labour costs.