Mostyn-Williams left Ashursts at the end of May with no new destination after what is believed to be a disagreement with management.
Now fellow Ashursts partners Clifford Atkins, Ian Goalen and rising star Anthony Ward have also resigned and are to join Mostyn-Williams at Shearmans at the end of their notice periods.
Mostyn-Williams has been appointed to the new post of head of European Acquisition Finance. Shearmans, which has more than 50 lawyers based in its London office, is moving aggressively into UK law.
Many of Mostyn-Williams' clients are US investment banks which are believed to want a single firm with dual US/UK law capability and may well go with him to Shearmans' London office.
Kenneth MacRitchie, head of Shearmans' English law practice, said: “Our clients are leading the globalisation of financial markets and are demanding advice under both New York law and English law.”
Ashursts managing partner Ian Nisse said that the firm remained committed to its banking and finance practice.
Commenting on the departures, a senior partner at a rival firm said: “This is the first sign of a chink in their [Ashursts] armour.”
Ashursts has appointed Justin Spendlove to replace Mostyn-Williams as head of its now six-partner banking team.