This month's impending merger vote by Dewey Ballantine and Orrick Herrington & Sutcliffe has been postponed as the two US giants push to tie up what would be one of the US' largest-ever legal unions.

Spokesmen at the two firms cited administrative difficulties for the delay, which has scuppered plans to integrate from 1 January as Dewey Orrick at the start of the combined firm's proposed financial year. The vote is now scheduled for the middle of January. However, decisions on the make-up of the new firm's management are at an advanced stage.

The 10-partner management board is set to be split equally between legacy Orrick and Dewey partners, with Dewey's London managing partner, Fred Gander, and Orrick's partner in charge of Europe, David Syed, in line to be the two European members on the board.

The combined board's membership would be finalised in a full partnership vote following a successful merger agreement.

The tie-up, which comes in a year that has seen a series of substantial mergers in the US, would be the largest legal tie-up since the 2004 union of Wilmer Cutler & Pickering with Boston's Hale and Dorr.