Cullinan, who was previously in charge of RBS's "bad bank" and had been praised for running down its unwanted assets more quickly than expected, was promoted to executive chairman of its Corporate and Institutional Bank (CIB) on Feb. 26.

A source familiar with the matter said that his departure is because of a disagreement over the implementation of strategy.

RBS, which is 79 percent owned by the British government, plans to shrink the investment bank, pulling out of 25 countries across Europe, Asia and the Middle East to leave it focussing on British operations.

That will result in the loss of thousands of jobs and about 60 percent of the investment bank's assets.

RBS has already shrunk its investment bank substantially since the 2007/09 financial crisis and last month said it would cut its assets on a risk-adjusted basis to between 35 billion pounds and 40 billion pounds by 2019, against 107 billion pounds at the end of last year.

"Let me be quite clear, this marks the end of the standalone global investment bank model for RBS," CEO Ross McEwan said at the time.

Cullinan was appointed as head of CIB on the same day as those plans were announced, so he agreed on the broad strategy, the source said, adding that the disagreement was over "how to get there".

One of the best paid executives at the bank, Cullinan was this month awarded shares worth 2.1 million pounds, vesting between 2016 and 2020.

RBS said on Monday that Chris Marks, chief executive of commercial and institutional banking, and Mark Bailie, chief executive of its non-core capital resolution business, will become co-CEOs of CIB.

(Editing by David Goodman)

By Steve Slater