Sainsbury’s (SBRY) this week reassured shareholders its legal challenge to the Competition & Markets Authority had not dented its “confidence” its mega-merger with Asda will go ahead, but nervous shareholders had other ideas.

News broke on Tuesday morning that Sainsbury’s and Asda had requested a judicial review of the CMA’s timetable, having had its request for extra time to review the huge amount of evidence on the deal rebuffed.

The supermarkets stressed their move was simply procedural to allow them to respond properly to the “large amount of material recently provided to us”, requesting an additional 11 working days over the Christmas period to adequately respond the to the material.

However, the unusual step of going to judicial review against the CMA raised market concerns that the retailers do not like the way the CMA’s investigation is developing.

“Whilst Sainsbury’s stresses this review is about process, rather than content, it still implies to us that Sainsbury’s is not satisfied with the combination of both,” said Bernstein analyst Bruno Monteyne. He pointed out the CMA has a “very standard” option to extend the process by eight weeks, meaning the body’s response that the request “would put our ability to complete the investigation by the required deadline at very serious risk” was “odd to say the least”.

“Given that Sainsbury’s and Asda have had more time than most to plan their path forward on the process and been in the driving seat for the proposed merger, one senses that they and their advisors have underestimated the job at hand,” added Shore Capital’s Clive Black. “Tactically, taking the referee to court rarely feels like good news in a deal process.”

AJ Bell’s Russ Mould warned: “Investors may begin to question Sainsbury’s management more strongly on the merits of pressing ahead with the tie-up. If the CMA rules against the deal then it is fair to say their credibility will be in tatters.”

Sainsbury’s shares sank 7.2% back to 275p on Wednesday – their lowest level since news of the merger first broke in April. The supermarket’s shares hit a multi-year high of 341.8p in August.