Frank Cushnahan, a former member of Nama's Northern Ireland committee, has suffered a heart attack and may not be able to testify before inquiries.
t is understood the prominent Northern Ireland-based businessman experienced ill health in recent days.
Mr Cushnahan stepped down from his Nama advisory role six months early, in November 2013, citing "personal reasons" and family concerns.
He subsequently became involved as an advisor with Pimco, a US investment firm which had expressed an interest in Nama's NI loan book.
The sale of the controversial 'Project Eagle' loan book to another US investment firm, Cerberus, is mired in controversy. It is now the subject of parliamentary inquiries on both sides of the border after claims that £7m was due to be paid to a Northern Ireland politician in the wake of the deal.