Shearman & Sterling's London arm has scored a lead role advising on the 20bn (£13.6bn) merger between Siemens and Nokia.

The US firm is advising Nokia on the deal, with London M&A partner Jonathan Coppin leading the team along with tax partner Iain Scoon and property partner Ian Nisse. The Shearman team worked closely with its German office.

The instruction is a coup for the firm's City arm, marking one of its biggest-ever transactions. Shearman is working with the Finnish mobile company's in-house department on the deal, with Nokia general counsel Carl Belding, assisted by legal M&A director Susanne Mattsson, also working on the transaction.

Shearman has acted for Nokia before, on its capital markets work. Other firms to have advised the telecoms giant include Simmons & Simmons, Denton Wilde Sapte and Jones Day.

Coppin told Legal Week: "We started work on this deal three weeks ago and it moved very quickly. It has massive global reach with around 100 different jurisdictions involved."

Clifford Chance (CC) is advising regular client Siemens on the negotiations, in what will also mark one of the magic circle firm's biggest deals of the year.

CC head of corporate Peter Charlton and corporate partner Joel Ziff is acting for the German engineering group.

The merger, which will close subject to regulatory approval on 1 January, 2007, will create a company of 60,000 employees.