A senior executive at Royal Bank of Scotland Group's Japan-based investment bank is expected to announce he is stepping down this week, before a possible sanction against RBS from Japanese authorities over the alleged manipulation of interbank lending rates, a person briefed on the matter said.

Ryusuke Otani, who heads RBS Securities Japan Ltd., a part of RBS's Japanese investment bank, couldn't be reached for comment. An RBS spokeswoman in Japan also declined to comment.