Clifford Chance has merged with Luxembourg firm Faltz & Kremer after months of wrangling with the Luxembourg bar.
The Lawyer first revealed the planned link-up in August, when talks were said to be at an "advanced stage".
But the merger was delayed until 1 January because of objections from Luxembourg's restrictive bar association - particularly regarding the name of the new firm.
Clifford Chance intended to call the firm Clifford Chance Faltz & Kremer, but the bar insisted it should be named Faltz & Kremer in association with Clifford Chance.
A Clifford Chance spokesman says: "We have had discussions with regard to the name with the Luxembourg bar and this is the name that was approved by them. It takes a while with the Luxembourg bar to do this sort of thing."
He says the agreed name will remain "for the foreseeable future".
Faltz & Kremer has 20 fee earners and specialises in finance, corporate, litigation and private banking. The firm's managing partner Christian Kremer will head the office.
The spokesman says Faltz's private banking expertise is "the key skill" it will bring to Clifford Chance. A Luxembourg office will also provide important backup on international finance transactions.
Clifford Chance lawyers will be placed in Luxembourg and Faltz lawyers will spend time at offices across the firm's European network.
Tim Herrington, the Clifford Chance partner who headed the negotiations with Faltz, says: "Luxembourg has developed as a major business centre, playing a key role in a number of our clients' cross-border European finance and corporate transactions.
"Faltz & Kremer have many synergies with our own practice."