By Matt Scuffham and Steve Slater
LONDON (Reuters) - Royal Bank of Scotland
Bostock, 52, has quit to rejoin Spain's Santander
McEwan has endured a rocky start since taking up the role of RBS chief executive in October, undermining his attempts to create a new customer-focused culture.
"The ... departure is a major inconvenience," one of RBS's biggest 40 shareholders told Reuters. "Recruiting senior people is a complicated and time-consuming process and to go through that, only to lose the hire within a matter of months, must be very disappointing."
RBS shares were the top fallers in a flat FTSE 100 <.FTSE>, down 2.7 percent by midday. Santander was up 0.4 percent.
Having been accused last month of closing down ailing small businesses too quickly in order to grab their assets, RBS faced more damaging publicity earlier this month when over a million customers were left without services by a computer failure.
The bank has appointed lawyers to study the small business allegations and admitted neglecting its IT systems for decades.
RBS was also one of several banks subjected to multi-million pound fines by European regulators following an investigation into the rigging of benchmark interest rates.
In addition, like other bank chiefs McEwan is grappling with a host of regulatory demands in the wake of the financial crisis of 2008-2009.
RBS, 82 percent state-owned, had said late on Tuesday Bostock was leaving. He is returning to Santander UK where he previously held positions including chief financial officer.
"I look forward to competing with him in the UK market as we strive to better the industry for our customers," McEwan said in a statement.
"He is a talented banker who brought a huge amount to our discussions with our regulators and our majority shareholder," McEwan said, referring to the government.
The departure comes as McEwan works on a critical strategy review which will be announced in February.
Bostock was expected to play a key role in the creation of a 38 billion pound ($62 billion) internal "bad bank" to hold troubled assets, which RBS committed to last month after the government decided against breaking it up.
His exit creates a management vacuum at RBS, which is also looking for a new head of its UK retail business, although the two vacancies offer a chance for McEwan to reshape the board.
RBS is running an international search for its new retail head and New Zealander McEwan, who previously ran Commonwealth Bank of Australia's
RBS has said Bostock will stay until his replacement is hired but that process could take months.
Given McEwan's recent appointment, Bostock may have opted to join Santander UK as the CEO role there could become available before the top job at RBS.
"In a way, we see it as an endorsement that the current CEO is here to stay - a factor that would definitely have weighed on Nathan's decision to leave," said analyst Chirantan Barua at brokerage Bernstein.
Santander said Bostock would be named chief risk officer and deputy CEO of its UK arm, which is due to be spun off and separately listed.
Santander UK Chief Executive Ana Botin has been tipped as a possible successor to her father Emilio as chairman of the Madrid-based group, maybe once the UK arm is floated.
"Is he going for the top job in Santander UK? It's common knowledge that the CEO of Santander UK is seen as one of the likely candidates for group CEO. In that case, Nathan ... definitely fits the bill," said Barua.
Bostock spent eight years at Santander UK and its predecessor Abbey National in a variety of roles, including as finance chief and head of financial markets.
He left Santander in 2009 to become head of restructuring at RBS, and oversaw the bailed-out bank shed 900 billion pounds from its bloated balance sheet. Bostock was appointed as RBS finance director on October 1, the same day McEwan took over as chief executive. ($1 = 0.6087 British pounds)
(Additional reporting by Chris Vellacott and Richa Naidu; Editing by David Holmes)