Nathan Bostock, who was appointed as group finance director at RBS on October 1, is understood to be in talks to join the Spanish bank Santander as its deputy chief executive and chief risk officer. He informed the RBS board last night of his intention to tender his resignation.
The timing of Mr Bostock's exit is unclear and any move to Santander would be subject to regulatory approval.
His departure is likely to be a further blow for the taxpayer-backed bank's chief executive Ross McEwan, who is conducting a comprehensive review of RBS's operations. Mr McEwan has had to deal with a succession of IT problems which last month caused hundreds of thousands of customers to be unable to use their debit cards or withdraw cash.
Last month the Financial Conduct Authority ordered a formal investigation by an independent expert into allegations that Royal Bank of Scotland systematically defrauded small business customers in distress. The Serious Fraud Office is also understood to be considering an investigation into the issue.
Mr Bostock's sudden announcement represents the second time in recent years that he has made a move to leave the RBS board after agreeing to take a new senior role elsewhere.
In 2011, he was due to leave his position as the head of restructuring and risk at RBS to run the wholesale operations of Lloyds Banking Group, but changed his mind after Antonio Horta-Osorio, the Lloyds chief executive, took sick leave.
Mr Bostock, a former Abbey National executive, was promoted to the finance director's post at RBS after the incumbent, Bruce Van Saun, left the UK to run Citizens, its US retail bank.
An RBS statement said: "His formal resignation is expected soon, but he will remain in his position to oversee an orderly handover of his responsibilities. Details on arrangements for his successor will be announced in due course."
Santander did not comment.