According to the Financial Times (FT), which cited unnamed sources familiar with the matter, Mr Buchan is expected to relinquish his non-executive role in March.
The newspaper reported that the departure of Mr Buchan, a former investment banker at UBS, will coincide with publication of the Financial Service Authorityâs (FSA) report on failings at the firm during the financial crisis.
Mr Buchan, who joined RBS in 2002 after working at UBS, is not implicated in the report, the FT was keen to emphasise.
He worked as chairman of the bankâs remuneration committee during the financial crisis until May of last year.
The FSA initially said the findings of its report into management failings at RBS would not be made public but is now looking to publish the the report in full in March.
RBS is believed to have paid PricewaterhouseCoopers more than Â£7 million to undertake research on its management failings.
By Jim Ottewill