Clifford Chance's Milan managing partner Nick Wrigley says his firm missed out because the deal began five years ago, before Clifford Chance's merger with US firm Rogers & Wells.
He adds: "We could have been expected to do at least the Italian part of the transaction, but we expect it's because we are the market leaders advising banks and sponsors who are trying to steal Enel's customers - that's the way the world works."
The Italian government is selling 3.6 billion shares, or 30 per cent of the company.
London-based partner at Skadden Arps Slate Meagher & Flom, Richard Muglia, and Gianni Nuncianti, partner at Italian firm Ughi e Nunziante's, are advising the Italian government.
Sullivan & Cromwell's London-based managing partner William Plapinger, and Francesco Ago, partner at Italian firm Chiomenti Studio Legale, are advising Enel.
Plapinger says: "It was a very complex transaction, coordinating between Italy and the US, and it was hugely successful."
Shearman & Sterling partner Bob Truehold and Roberto Cera, partner at Milan firm Bonelli Erede Pappalardo, are advising underwriters Merrill Lynch and Medio Banco.
Truehold says: "We've been involved in this transaction since 1994.
"This is a very complicated deal because of change in the regulatory regime and because it was such a huge transaction."