Helou was brought in by Barclays' investment banking arm, Barclays Capital, from Rothschild in 2010 to lead the division which advises banks and other financial institutions in the region on mergers and acquisitions, fixed income and equity capital markets transactions.

Dubai-based Helou left the bank few weeks ago, the sources said, speaking on condition of anonymity as the matter is not public. Both Barclays and Helou were not immediately available to comment on why he left and his plans for the future.

Helou originally moved to Dubai in 2006 to set up Rothschild's regional office. At Barclays, he worked on several prominent deals including Qatar Holding's 2010 purchase of a stake in Santander's Brazilian unit, and Kuwaiti lender Burgan Bank's purchase of EFG Eurobank's Turkish unit, the sources said.

He was also involved in the planned merger of Qatar's Al Khaliji Commercial Bank and International Bank of Qatar, a deal which stalled in 2011 after the parties failed to agree terms.

Middle Eastern deal activity has been picking up after a subdued period. Cash-rich Gulf Arab sheikhs and governments are buying European assets, lured in part by attractive valuations due to weak markets.

Barclays topped the Middle Eastern M&A fee rankings for completed deals in 2012, according to Thomson Reuters data, raking in 9 percent of the overall fee pool in the segment.

The Middle East, home to some of the world's largest sovereign wealth funds, will be a key area for investment banks including Barclays in coming years, Barclays' global vice-chairman for investment banking, Makram Azar, told Reuters in an interview last year.

(Reporting by Dinesh Nair; Editing by Andrew Torchia)

By Dinesh Nair