Sainsbury’s and Asda’s potential £15bn mega merger is facing ­opposition from rival supermarkets over fears it will harm customer choice by shrinking competition in the UK grocery market. 

Britain’s second biggest supermarket is set to announce a colossal tie-up with Asda today, creating a retail giant with £50bn sales, more than 2,000 stores, and a greater market share than industry leader Tesco. 

The Daily Telegraph understands Walmart, the US retail giant that snapped up Asda in 1999, would emerge as one of the biggest shareholders in the group with a 40pc stake if the deal goes ahead. 

The Sainsbury’s and Asda brands will also be kept as part of the deal, it is understood, with Sainsbury’s boss Mike Coupe becoming group chief ­executive and Roger Burnley remaining in charge of Asda. 

However, the proposed deal is facing intense pressure from politicians, unions and competitors over fears it may spark thousands of job losses, drive down choice and squeeze suppliers. 

The competition watchdog is being urged to launch an immediate inquiry into the deal, and force Sainsbury’s and Asda to offload stores in areas where the group could gain a monopoly. 

“It is not good for customers to have that much concentration in the market,” a source at a rival UK supermarket said.

“There are a number of ways in which customers could be affected and the question is going to be ‘are there going to be separate prices for Asda customers and Sainsbury’s customers if the goods are coming from the same place with different branding?’

“I think there will also be a lot of suppliers thinking ‘will I still have business with this company? And if I am going to have business then I will get asked to reduce my prices?’

“(Sainsbury’s-Asda) synergies will be in the buying, and its suppliers that will face the toughest deal of all.” Such a tie-up would see Sainsbury’s and Asda seize nearly a third of the UK market, trumping Tesco’s 28pc market share. 

Market share: How the grocery sector stacks up

However, analysts said the enlarged brand’s dominance could be diminished if it is forced to offload stores to satisfy competition concerns. Bruno Monteyne, the Bernstein analyst, said the deal could spark cost savings of £600m, with the majority of the synergies coming from renegotiating terms with suppliers.

It is understood the combined firm would invest to drive down prices in a bid to coax bargain-hunting customers away from Aldi and Lidl. However, Clive Black, the Shore Capital analyst, said the cost savings could also be targeted at the supermarket’s back office functions, putting hundreds of jobs under threat.