Peter Rees QC, the former legal chief of Shell, has joined Thirty Nine Essex Street as a counsel and commercial arbitrator, Legal Week can reveal.

Rees, the one-time head of Norton Rose's global dispute resolution practice, moves to Thirty Nine Essex this week, becoming the chamber's 40th silk.

His departure from Shell in January was a shock to a number of the oil major's advisers, coming just three years after he joined the company from litigation heavyweights Debevoise & Plimpton.

In February, sources close to Rees confirmed he had decided to return to international arbitration rather than move into another senior in-house role.

"I always had the view that when I left Shell I would return to arbitration," Rees told Legal Week. "Prior to leaving, the only thing I hadn't got clear in my mind was whether I would do commercial work as well."

"I was very lucky to have a number of options both at law firms and chambers. Thirty Nine Essex Street is the perfect home for my practice and my aim is to split my time between arbitration and commercial work, with no more than 75% spent on arbitration."

Rees also said he wanted at least half of his cases to be general commercial matters, despite his specialism in energy and construction.

"I don't want to be pigeon-holed into either practice," he commented. "I have expertise in both areas, but I would like at least half of my work to be general commercial disputes."

Rees' departure from Shell prompted much speculation in the City, given the company's recent profit warnings, the on-going European Commission investigation into oil price-rigging and the arrival of new CEO Ben van Beurden.

"I had a fantastic time at Shell, and I wouldn't have missed it for the world," said Rees, while declining to comment on the circumstances of his departure from the company. "Donny is a great guy to fill the role, and I would like to think I've left the legal team in great shape for him to pick up and run with it."

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.

His new home, 39 Essex Street, has pursued an expansive international strategy in recent years, opening in Singapore last year and announcing plans to open in Kuala Lumpur last month.

Last June, the set appointed new co-leaders of the chambers, with Neil Block QC and Stephen Tromans QC taking over from Leveson Inquiry lead counsel Robert Jay QC, who joined the High Court Bench last summer.