Orange chief executive Tom Alexander will take the helm at the new firm and T-Mobile chief executive Richard Moat will become chief operating officer.
The joint venture will be the largest mobile network provider in the UK, with a 37 per cent market share, eclipsing rivals O2 and Vodafone.
The European Commission ruled the firms had offered enough concessions to give the deal the green flag.
Objections by the Office of Fair Trading had already been withdrawn, after early indications it would demand the right to launch its own investigations.
Orange and T-Mobile agreed to sell 25 per cent of the valuable 1800MHz range radio waves to its rivals. The range is able to support super fast fourth generation mobile internet.
Analysts say the spectrum has “serious value” but do not expect it to raise anywhere hear the £20bn raked in when the government sold the original 3G spectrum. Orange added the sale will not take place for at least two years.
A deadline has been set for the company to offload most of the spectrum by 2013, with a second smaller chunk being released in 2015. A private sale will take place, rather than the open auction held for the 3G network.
The company accounts of T-Mobile and Orange will merge as soon as April but the distinct brands will remain for at least 18 months. Minimal consolidation, such as closing adjacent shops, may occur before then. Both firms declined to comment on how many staff will be lost when full consolidation occurs.