Freshfields is acting for Swedish drugs giant AstraZeneca in its merger with Swiss rival Novartis.
Corporate partner Julian Long is leading the Freshfields team. It is thought that he will be dealing mainly with regulatory issues but Freshfields says it is too early to say exactly what these will be.
Novartis has not yet decided which law firm it intends to use for the merger. But sources says it is thinking of appointing two UK firms to handle the work, believed to be Herbert Smith and Eversheds. The source adds that Novartis has been using a UK practice to advise on the deal, but a conflict of interest has arisen.
Rowe & Maw, which is regularly used by Novartis, has been conflicted out as it also works for AstraZeneca.
The Swiss headquarters of Novartis is thought be considering a US firm to handle the merger on worldwide matters.
The new company will be called Syngenta. It is expected to have a market capitalisation of over £10bn when its shares start trading next year.