Before the deal can be given a green light, economists at the Competition and Markets Authority (CMA) are expected to spend months poring over a map of the combined group's store network.
Lib Dem leader and former business secretary Vince Cable has urged for an investigation to be carried out by the competition watchdog.
Shadow business secretary Rebecca Long-Bailey 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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Together their combined market share would slightly surpass Tesco's market share, which would drive buying synergies, it said in the note.
Sky News broke the story about the possible merger on Saturday, when it said that the two companies were in negotiations, with an announcement to be made as early as this coming Monday.
Retro 90s fashion is a huge trend on the high street at the moment but that nostalgia has now spilled over into food retail as the supermarket giant Sainsbury's attempts a blockbuster deal that will result in it seizing back the United Kingdom grocery crown it surrendered to Tesco in 1995.
"GMB will be making sure the voices of supermarket workers are not lost amidst all the talk of mergers and acquisitions".
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Yet the deal makes a lot of sense - especially if we assume it is Walmart, the United States giant that owns Asda, effectively waving a white flag on the United Kingdom market, and deciding it would prefer to throw in its lot with another operator.
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".
It would not be the first time Sainsbury's has acquired another high street giant. In 2016 Sainsbury's purchased general merchandise retailer Argos for 1.1 billion pounds.
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.
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With US output surging, some analysts warn that the 20 per cent climb in Brent prices since February is starting to look overdone. Said Greg McKenna, chief market strategist at futures brokerage AxiTrader, "The market does look a little toppish".
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.