Lawyers played a "focal role" in the US$166bn Citicorp and Travelers merger claimed to be the largest ever according to James Healy, London partner at Skadden Arps Slate Meagher & Flom, the US firm which is advising Travelers.
Healy said that the lawyers' role was raised by the fact that the deal took place with little input from investment bankers.
The Skadden Arps team, described as "substantial" by Healy, is led by New York partner Kenneth Bialkin also a board member at Travelers.
Healy said that his firm's London and Brussels offices had been given the job of co-ordinating the steps necessary to gain the deal worldwide regulatory approval, and this could take up to a year, reducing the likelihood that Travelers will instruct a UK firm.
If the deal goes through, it is likely to cement Skadden Arps' position in the US as the leading legal adviser on big-ticket corporate finance work. The firm has already been involved this year in 31 deals worth over US$43bn.
Citicorp also has a strong relationship with the firm it has instructed, Shearman & Sterling. Its general counsel, Jack Roche, is an ex-Shearman partner, as were his four predecessors.
It is understood neither bank has yet instructed a UK firm.