The Competition and Markets Authority (CMA) has launched an investigation into whether the merger would “result in a substantial lessening of competition” in the UK.
Earlier this year, the US internet giant was the lead investor in a US$575mln funding round undertaken by Deliveroo, which said it would use the money to improve its technology.
The news sparked controversy, with Labour’s deputy leader Tom Watson claiming Amazon was just trying to access more consumer data.
In its statement on Wednesday morning, the CMA ordered the two firms to keep their activities separate while the investigation is ongoing.
The current deadline for a decision on the first phase is 11 December.