Brunswick Group is to provide global corporate and investor relations advice, following a six-way pitch.
Headed by MD James Bradley in the UK and partner Cindy Leggett Flynn in the US, the brief will include work on integration plans for KeySpan.
Brunswick replaces nine-year incumbent Citigate Dewe Rogerson.
Bell Pottinger Public Affairs was brought in at the end of last year to provide European and UK support. It replaced Central Lobby Consultants.
Both agencies report to director of corporate affairs George Mayhew.