The Competition and Markets Authority (CMA) has launched an inquiry to determine if Amazon's investment in Deliveroo could breach competition rules.
The CMA had earlier issued an Initial Enforcement Order against the two firms after Amazon bought a minority stake in the delivery company. It has now said it will examine if the purchase could see a "substantial lessening of competition", with a decision on the first phase of the investigation expected by 11 December.
The investment followed the closure of Amazon's own food delivery wing, Amazon Restaurants UK. The concept was wound up in December 2018, two years on from its launch.
A spokesman for Amazon previously said: "We believe this minority investment will enable Deliveroo to expand its services, benefiting consumers through increased choice and creating new jobs as more restaurants gain access to the service."
A spokesperson for Deliveroo added: "Deliveroo and Amazon have been working closely with regulators to obtain regulatory approvals.
"There are a number of major companies within the restaurant food delivery sector and this investment will enable Deliveroo to expand, innovate and, we believe, will enhance competition.
"This investment will help create jobs, help restaurants to grow their businesses and will improve choice for consumers."