Cleary Gottlieb Steen & Hamilton London financial regulatory partner Bob Penn (pictured) is leaving the firm after less than two years, to rejoin Allen & Overy (A&O).

Penn quit A&O for Cleary in September 2016 after nine years as a partner. His return to the magic circle firm bolsters its regulatory team to six partners in London.

Penn joined A&O's regulatory practice in 1999 and made partner in 2007. Before leaving for Cleary, he was head of the UK firm's non-contentious financial services regulation practice, advising banks, asset managers, market infrastructure providers and other financial institutions on a wide range of non-contentious regulatory issues.

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 is known for advising HSBC on its UK bank ring-fencing project, which was completed in May this year. As part of the project, Penn gave evidence to parliamentary commission on banking standards in relation to proposals for the ring-fencing of retail banking activities in the UK.

Damian Carolan, who heads A&O's financial services regulatory practice in London, said:  "We are very excited to have Bob back at A&O. His depth of experience is an invaluable addition to our team at a time when the volume, complexity and breadth of financial regulation has increased exponentially. This is driving continued investment in the team to continue to support our financial services clients in facing sophisticated and evolving challenges."

News of Penn's return to A&O comes after the firm hired banking partner Ben Regnard-Weinrabe from Paul Hastings earlier this year.

A&O is currently in discussions about a potential tie-up with US firm O'Melveny & Myers in a bid to crack the US market.

A Cleary spokesperson said: "We wish Bob all the best for the future. Our financial services regulatory practice is an important global and cross-border practice. We will continue to support our financial institution clients across the world, with expertise from our offices in the Americas, Asia, London and the rest of Europe."