Latham & Watkins and Cleary Gottlieb Steen & Hamilton have scored the prize mandates on one of the biggest pharmaceutical deals of the year, as Actavis has agreed to buy Botox-maker Allergan for $66bn (£42bn) in cash and shares.

The sale is expected to close in the second quarter of 2015, creating one of the world's 10 biggest pharmaceutical companies with annual revenues close to $23bn (£14.7bn).

Allergan, which is expecting to generate $1.8bn in cost savings through the tie up with Actavis. had previously rebuffed takeover offers from both Canadian firm Valeant Pharmaceuticals and activist investor William Ackman.

Cleary is serving as Actavis' lead legal advisor. The firm's M&A team is being led by partners Victor Lewkow and Paul Shim, with New York debt finance, securities and capital markets, employee benefits, tax and litigation partners also acting on specialist areas.

The firm is also acting as European antitrust counsel, with partners Nicholas Levy and Christopher Cook heading Cleary's team in Brussels.

Latham is representing Allergan in the transaction with a corporate team led by partners Cary Hyden, Paul Tosetti and Michael Treska.

Advice has also been provided on antitrust matters by Latham partners Karen Silverman and Joshua Holian in San Francisco.

Stikeman Elliott is acting as Canadian counsel to Allergan, fielding a team including M&A partners John Leopold, Beno