Orange and T-Mobile lawyers are expected to close the deal on the UK joint venture project at a meeting at the end of April, and set up a new business with Companies House.
It will be the first time they can share confidential financial and customer information.
Following EC approval earlier this month, the pair insist the two brands will co-exist for 18 months following the foundation of a new trading company.
It is expected a roaming agreement between the two will be set up shortly afterwards. The consolidation of assets, including staff, will be “much quicker than anyone expects”.
A source said: “People will be amazed at how quickly the integration process happens. It is a monumental amount of work but things will happen and be completed quickly.”