The Competition and Markets Authority (CMA) could stop Tesco's proposed £3.7bn takeover of Budgens and Londis owner Booker, according to a retail body that represents 33,500 local shops.
"The CMA could either stop or put conditions on the deal," James Lowman, chief executive of the Association of Convenience Stores told City A.M.
"The deal was a real shock, no one saw it coming. We expect the CMA will make a proper and informed analysis based on a detailed understanding of the market. We will assist them in any way we can."
On Friday, the two retail giants announced they will combine operations in a share and cash deal worth £3.7bn.
Tesco boss Dave Lewis said he hoped the deal would complete by the end of this year or early next year.
Ocado chairman Lord Stuart Rose said the watchdog could scrutinise the deal because it may result in majority of small shopkeepers being supplied by Britain's biggest supermarket.
“The Competition and Markets Authority (CMA) will be all over it like a rash, and they should be. The small shopkeepers are now effectively going to be supplied by Tesco,” Lord Rose told the Sunday Times.
Both Tesco and Booker played down competition concerns when the deal was announced on Friday.
Booker boss Charles Wilson said: "We think this is a pro-competition deal."