The Competition and Markets Authority (CMA) is concerned the deal could result in worse terms for restaurants using either company due to the similarity of their services.
Earlier this month Just Eat said it was ‘committed to demonstrating to the CMA that the market is, and will remain, competitive following completion of the proposed transaction’.
But the CMA was not satisfied these conditions were met and has launched an investigation led by an independent panel.
The deadline for the final decision is 2 November this year.
Just Eat processed a total of 88.1m orders in the UK in 2016.
Though online delivery companies such as Deliveroo and UberEATS offer similar services, the CMA sees them as less direct competition as they target largely dine-in restaurants brands.