Elite New York firms Sullivan & Cromwell and Wachtell Lipton Rosen & Katz have scooped the lead roles on the biggest M&A deal in the financial services sector so far this year, as US bank Wachovia acquires mortgage lender Golden West Financial for $25.5bn (£13.7bn).

Wachovia, the fourth-largest bank in the US, has instructed Sullivan, which is fielding a team out its New York office led by high-profile firm chair Rodgin Cohen and financial institutions partner Mitchell Eitel. Benefits partner Marc Trevino and tax partner Andrew Mason are also advising.