Peter Rees QC, the former head of Royal Dutch Shell's legal team, is to return to international arbitration, Legal Week has learned.

Sources confirmed that Rees (pictured), who is currently taking a month's leave following his departure from the oil giant, has decided against another in-house role, but instead plans to return to either a law firm or the bar.

Last month, Rees stepped down as legal director of Shell, just three years after joining the company from Debevoise & Plimpton.

The shock move, which had not been trailed before a 10 January announcement, has prompted much speculation in the City, given the recent profit warnings by Shell, the on-going European Commission investigation into oil price-rigging and the arrival of new CEO Ben van Beurden.

One source close to the matter said: "Only three people at Shell know the reason behind his exit: Peter, Ben and [chief human resources and corporate officer] Hugh Mitchell."

Following Rees' departure, Mitchell took on responsibility for the role, with support from company secretary and general counsel corporate Michiel Brandjes.

Earlier this month, Shell confirmed former project and technology group general counsel Donny Ching had been installed as the company's new legal director with immediate effect.

Rees began his legal career in 1979 at Norton Rose, later going on to head the firm's dispute resolution practice in 1987 and joining the executive committee in 1997.

He moved to Debevoise in 2006, at the time becoming the New York firm's only UK-qualified litigation partner in London, where he quickly brought all of the firm's advocacy in-house.

Rees then took silk in 2009 before joining Shell a year after.

In an interview in 2011, Rees called his seat on Shell's executive committee "a huge selling point – I have spent my life advising businesses but you never get access on the commercial side".

Legal Week hosts a series of flagship events for leading corporate counsel every year – click on the following links for more information.