The Competition and Markets Authority (CMA) announced on Friday it has now entered phase 2 of the investigation, with a decision to be released “as soon as is reasonably practicable”.
"The CMA has referred the anticipated acquisition by Amazon of a minority shareholding and certain rights in Deliveroo for an in-depth investigation," said the regulator.
The US online shopping giant was the lead investor in a US$575mln funding round undertaken by Deliveroo earlier this year, which the CMA said could reduce competition in the UK market.
The two companies were previously ordered to keep activities separate while the investigation is ongoing.
No details on the possible size of Amazon's stake have been published.