London: The U.K.’s competition regulator opened an in-depth probe of Amazon.com Inc’s purchase of a minority stake in UK food start-up Deliveroo, after the companies failed to submit remedies to assuage competition concerns.

The Competition and Markets Authority began a review of Amazon’s move to secure part of the food delivery service in October for around $500 million and blocked the companies from integrating any further. The regulator set a June 11 deadline to rule on the deal.

Many were surprised by the CMA’s decision to investigate the transaction because the agency doesn’t typically review minority acquisitions. The move may have been down to growing fears about monopolies in Big Tech that have been allowed to go unchecked in the past.

“We are confident that we will persuade the CMA of the facts that this minority investment will add to competition, helping restaurants to grow their businesses, creating more work for riders, an increasing choice for customers,” Deliveroo said in an emailed statement.

Amazon representatives didn’t immediately respond to requests for comment.