Allen & Overy's (A&O) head of non-contentious financial services regulation has joined US firm Cleary Gottlieb Steen & Hamilton.

Bob Penn joined Allen & Overy's regulatory practice in 1999 and became a partner in 2007.

While at A&O Penn led the magic circle firm's work on the European financial services reform agenda, including advising on revisions to capital requirements, the introduction of recovery and resolution plans and retail ring-fencing for banks. He also played a role in drawing up changes to derivatives markets.

Recently Penn advised HSBC as A&O picked up the lead advisory mandate for the bank, alongside Norton Rose Fulbright, as the UK's biggest banks prepare for incoming regulations on ring-fencing their high street operations.

The ring-fencing rules, recommended by John Vickers in 2011 and set to come into effect in 2019, are intended to protect the public by ensuring that if a bank were to crash its retail banking operations would be unaffected by problems in its riskier investment banking business.

Penn will join Cleary after serving out his notice period at A&O.His departure is one of an ongoing trend of London-based magic circle partners moving to US firms.

In October 2015, A&O's City capital markets partner Thomas Jones left to join the London office of US firm King & Spalding.

Jones joined the magic circle firm as an associate in 1995 from Cravath Swaine & Moore. He made partner in 2000 and spent five years in Hong Kong before returning to London in 2008.

In November 2015 Linklaters restructuring heavyweight David Ereira was hired by Paul Hastings to boost its City office.

He joined Linklaters in 2007 as a partner from Freshfields Bruckhaus Deringer. He worked for Freshfields for 17 years between 1990 and 2007.

Cleary rarely hires partners in the City. In 2012 the firm hired former Stephenson Harwood chief executive Sunil Gadhia.

He joined the firm's London litigation practice, which launched in 2010 with the hire of finance litigation partner Jonathan Kelly from Simmons & Simmons.