Linklaters and Slaughter and May are handling the merger of French management consultants Cap Gemini with Ernst & Young's consultancy arm.
Corporate partner Richard Godden is leading the Linklaters team, which is advising Ernst & Young on UK and French matters.
Cleveland firm Jones Day Reavis & Pogue is representing Ernst & Young on US matters.
Slaughters company partner Christopher Saul is advising Cap Gemini on UK matters.
Sullivan & Cromwell is acting for it on US issues, and Brendin Prat is handling it from the French law side.
Godden says that because Ernst &Young is an international organisation the deal is very complex and requires legal expertise in different jurisdictions.
He says: "We have to handle the co-ordination of related but separate events in different countries.
"The key issues in a business like this are human, because the partnership is being sold. Partners meet it with mixed emotions."
Garth Bray, Sullivan & Cromwell corporate assistant, says: "These guys are constantly being head-hunted. The trick is to put together a deal that is smooth and attractive so that none of them leave."
The deal has been signed in the US, UK, France, Germany, Italy and Spain. Lawyers are currently negotiating with partnerships in The Netherlands, Belgium, Australia, New Zealand, Sweden, Denmark, Norway and Finland.
Other branches will be approached when these deals have been completed.