Orrick Herrington & Sutcliffe and Dewey Ballantine have explained the logic behind their merger attempt to create a US giant with almost 1,500 lawyers and revenues of nearly $1bn (£535).

The firms, which will vote on the proposed tie-up in mid-December and are aiming for the merger to go live on 1 January, will practise under the banner Dewey Orrick.

The vote will see partners at both firms given the chance to veto the proposal, although partners involved in the negotiations have admitted the tieup is "a done deal".

Orrick managing partner Ralph Baxter underlined that building their New York and London bases was the main motivation behind the merger. The New York office of the merged firm would boast nearly 500 lawyers, 285 from Dewey and 211 from Orrick, while the London arm would have 98 lawyers, with Orrick contributing 57 of them.

Baxter told Legal Week: "Our single most pressing priority for the last two years has been to build our New York practice and if the merger goes through, no-one can offer that kind of quality both in New York and on the West Coast."

Orrick hopes to cross-sell its New York offering to existing clients such as Ebay, Citigroup, Goldman Sachs and Deutsche Bank.

As well as its hub on the West Coast, Orrick offers Dewey a substantial international network, significantly in Europe and the Far East.

In the past few years, Orrick has achieved significant growth, with notable acquisitions including a chunk of defunct Paris firm Ram-baud Martel and the London, Moscow and China offices of Coudert Brothers.

Baxter and Dewey chairman Morton Pierce, who have brokered the deal, will jointly head the merged firm, with Baxter taking on a managing partner role and Pierce remaining heavily involved with his corporate practice.

The new firm will also have a substantial management board, with Pierce and Baxter set to appoint leaders of every office and practice group.