The in-house team of six at BT had just a couple of weeks to draw up the details of its £12bn takeover of US telecommunications company MCI to form the new company Concert.

Led by BT's chief legal advisor Colin Green, the team of five corporate and one regulatory lawyer split between London and New York for the deal with Green shuttling between the two.

Jeff Fisher, on the London team, said: “The team in New York worked on the merger details while in London we sorted out the structural details of the new company.”

BT also instructed Linklaters & Paines for the UK aspects of the deal. The Linklaters team was led by partner Donald Williams and included tax partner Mark Kingston and corporate partner David Barnes. In the US, BT used a Shearman & Sterling team headed by John Madden in New York while Jeff Lawrence also worked on the deal from Shearmans' London office.

Fisher said MCI did not make much use of its in-house lawyers: “Only its senior lawyer Mike Salisbury was in on the deal from the start.”

MCI instructed Simpson Thacher & Bartlett in New York and Freshfields in London. The Freshfields team included regulatory Brussels-based partner Rachel Brandenburger, tax partner Timothy Ling and employment partner Simon Evans. It was led by head of Freshfields corporate department Gavin Darlington.