Current President Nobuyuki Hirano will become chairman while Mike will also continue to serve as CEO of the banking unit, said the source, who requested anonymity because he was not authorised to discuss the matter.

An spokeswoman for MUFG, Japan's largest lender, said nothing had been decided.

Joining what is today's MUFG in 1979, Mike spent about half of his four-decade career outside Japan and became CEO of the core unit MUFG Bank last year following his predecessor's sudden resignation for health reasons.

Mike is taking over the helm of the financial conglomerate with over 300 trillion yen ($2.7 trillion) in assets.

Hirano, who led the banking group since 2013, has been pushing the expansion of its overseas operations, as its western peers shrank their businesses in the wake of the global financial crisis a decade ago.

MUFG invested $9 billion in Morgan Stanley in 2008 and now has a 24 percent stake. Many inside and outside the bank said Hirano, fluent in English, played a major role in the Japanese lender's successful tie-up with the Wall Street bank in the following years.

In Asia, the bank has been making a string of deals, including acquiring a controlling stake in Thailand's Bank of Ayudhya [BAY.BK] in 2013.

(Reporting by Taiga Uranaka and Taro Fuse; Editing by Chris Gallagher and Robin Pomeroy)