Freshfields Bruckhaus Deringer and Herbert Smith are on the verge of securing a successful completion to the epic merger talks between P&O Princess Cruises and Carnival Corporation – creating the first ever company to be dual-listed in London and New York.

The long-trailed tie-up of P&O and Carnival – advised by Freshfields and Herbert Smith respectively – is also the first time an announced offer for a company has been converted into a dual-listed entity.

Slaughter and May client Royal Caribbean withdrew its £3bn merger offer for P&O on 25 October, leaving the field clear for Carnival's rival bid of £3.5bn.

The P&O Princess and Carnival shares are expected to continue to be included in the Ftse 100 and the Standard & Poors 500 indices respectively.

The partial share exchange offer will allow P&O Princess shareholders to retain an interest in the combined enterprise by keeping their UK listed shares, or by exchanging them – subject to an overall limit of 20% of P&O Princess shares – for US-listed Carnival shares.

The merger, which requires shareholder approval, is expected to go ahead early next year. Depending on the extent to which the partial share offer is accepted, 21%-26% of the economic and voting interest in the merged company would be represented by P&O Princess shares.

Herbert Smith corporate partner Anthony Macaulay, who led the deal, said: "This has been an immensely complex and long-lasting transaction that has broken new ground in several ways."