A partner vote on the planned merger of Allen & Overy and Shearman & Sterling is set to start today, which will decide whether the transaction will proceed.

Voting will be completed on Friday 13 October, with the outcome to be announced shortly after. If partners approve the deal, Allen & Overy and Shearman & Sterling will – subject to customary closing conditions – come together to create a transatlantic behemoth with just under 4,000 lawyers and about $3.4bn in annual revenue. The combined firm will be known as A&O Shearman.

Will Dejonghe, senior partner at Allen & Overy, said: “Preparations for the proposed merger are going well and we have received great feedback from clients and colleagues. Over the past few months, partners and teams from both firms have been meeting and building relationships, and the excitement about the opportunities for the merged firm is palpable."

He added: “This response has only strengthened our belief in the transformational nature of this merger. We are both highly confident we will obtain the approval required to bring to fruition this unparalleled combination.”

The firms also announced today that they have completed a number of key transaction milestones, including financial and operational due diligence, antitrust filings and approval for all required adjustments to retirement and pension programmes.

Adam Hakki, senior partner at Shearman & Sterling, said: “This transformative transaction will bring a seamless offering across geographies to tackle clients’ most complex issues. We have made significant progress since announcing the combination, and our clients have expressed enthusiasm for the combination and what A&O Shearman will be able to deliver. Our partners and colleagues are very supportive and eager to launch the combined firm.”

Since the merger was first announced back in May, both firms have seen a number of partner departures. A&O’s global managing partner Gareth Price resigned in July, citing personal reasons. Shearman, meantime, has lost partners to firms including Ashurst, Freshfields Bruckhaus Deringer, Cleary Gottlieb Steen & Hamilton, O’Melveny & Myers and Proskauer Rose.

The proposed combination came after Shearman and Hogan Lovells called off merger talks in March, saying it was “not in the best interest of either firm”.