Last week Carphone dismissed talk of a buy-out as speculation but have since confirmed that a deal has been struck.
Mobiles.co.uk was founded in Watford in 1995 and turns over in excess of £9 million per year.
It is the only online retailer to have been granted entry to Oranges Federated Dealer Club.
Last year, it overhauled its content management system and website. It offers instant online credit checking decisions across all five networks, and connects direct with Orange, O2, 3 and T-Mobile.
Carphone shut its OneStop PhoneShop online retail operation last month.