Ashurst is advising the banks Citigroup, ABN Amro, Lehman Brothers and Morgan Stanley, while A&O is acting for Imperial Tobacco.
To fund the bid Imperial Tobacco will launch a £5.4bn rights issue within a year, with Ashurst and Allen & Overy to reprise their roles when the time comes.
Nicholas Holmes, Ashurst partner on the Imperial deal, said: “The rights issue is likely to be the biggest in UK corporate history; it’s very complicated and interesting. For us it’s a fantastic deal to have been involved on.”
Norton Rose and Garrigues have cemented their working relationship by acting for Spanish tobacco company Altadis together.
Norton Rose corporate partner Nick Adams advised on English law matters, with Garrigues concentrating on the Spanish side.
A&O’s team is being led by London-based corporate partners Jeremy Parr and Helen Harrison-Hall and Spanish-based partners Inigo Gomez-Jordana and Juan Barona.