The Competition and Markets Authority (CMA) said it had decided to delay the publication of its report to give it enough time to “reach a fully reasoned final decision” on the £12bn merger.
An inquiry group made up of CMA panel members was due to report back by 5 March, but this timetable has now been extended to 30 April.
The inquiry group’s chair, Stuart McIntosh, said the decision to extend the probe was taken because of the scope and complexity of the investigation and the need to consider submissions made by the main parties as well as third parties.
Sainsbury's and Asda issued a joint plea late last year for more time to respond to concerns over the takeover.
If given the green light, the newly-formed group would be set to replace Tesco as Britain’s largest supermarket chain, commanding almost £1 in every £3 spent on groceries.
However, it is likely the CMA’s approval for any deal will required the sale of Sainsbury’s and Asda stores in areas of overlap to competitors, with current estimates ranging from between 70 to 300 stores.
Read more: Sainsbury’s suffers Christmas sales slowdown
The managing director of frozen foods giant Iceland told the FT this week that the supermarket would be interested in snapping up Asda or Sainsbury’s stores.
"We are looking at everything for sure," Iceland's managing director Richard Walker said. "Food retail drives footfall and we are the only supermarket that is opening on retail parks... We are looking at every retailer, trying to pick off as much as we can."