According to WikiLeaks, he told US politicians that RBS directors had failed in their "fiduciary responsibilities" and had not carried out proper due diligence before the ill-fated purchase of Dutch bank ABN Amro.
He is said to have made the remarks in September last year at a meeting, nine months after his appointment.
Meanwhile, Labour's former City minister Lord Myners, who was instrumental in securing Lloyds' takeover of HBOS, has called for the big banks to be broken up.
He said: "The future lies in less monolithic institutions.
"The banking commission must give proper consideration to splitting one or both of Lloyds Banking Group and the RBS."