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(Reuters) - Britain's competition regulator is launching an in-depth investigation into Amazon.com Inc's AMZN.O purchase of a stake in online food delivery group Deliveroo after the companies failed to assuage its initial concerns.

Amazon led a $575 million fundraising by Deliveroo in May and the probe could disrupt the e-commerce giant's efforts to compete with Uber Eats UBER.N, Just Eat JE.L and Takeaway.com TKWY.AS in food delivery.

The Competition and Markets Authority (CMA), which launched an initial probe in June, said bit.ly/2EY4Ete on Friday it had decided to move to a "Phase 2" investigation as Amazon and Deliveroo had not addressed its concerns before a Dec. 18 deadline.

Earlier this month, the CMA said Amazon’s investment into Deliveroo in its current form could harm competition in restaurant food delivery and online convenience grocery delivery.

The deal would give Amazon more say in the running of Deliveroo, which has grown rapidly in the last six years to more than 100 towns and cities across Britain.

Neither Deliveroo nor Amazon have not disclosed Amazon’s stake in the company.

Seattle-based Amazon defended the deal on Friday, saying Deliveroo should have broader access to investors.

Deliveroo said it was working with the CMA and the deal would “add to competition, helping restaurants to grow their businesses, creating more work for riders and increasing choice for customers”.

The CMA in April blocked supermarket group Sainsbury's SBRY.L 7.3 billion pound ($9.6 billion) agreed takeover of Walmart-owned WMT.N rival Asda.