McEwan is to make the announcement tomorrow, according to Sky News, despite the confidence extended to him from City investors and his board.
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The donation of half of the £1m allowance to charity will be disclosed alongside RBS's annual results tomorrow, which are expected to show another annual loss at the bank.
The remaining 50 per cent of McEwan's allowance will be held by him in the form of RBS shares.
The New Zealander receives a pension contribtuion worth several hundred thousand pounds per year, as well as a long-term share award, in addition to the basic salary of £1m. Still, McEwan's salary is far lower than other bank bosses.
The Treasury and UK Financial Investments have been briefed on McEwan's decision. The Treasury retains a 73 per cent shareholding in RBS, valued at more than £20bn.
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It comes days after chancellor George Osborne named Oliver Holbourn as the man in charge of offloading government assets. Osborne has said he wants to sell £25bn of RBS shares alone by the end of the decade.
Last month the bank announced it will take a £2.5bn hit on its 2015 profits, thanks largely to a £1.5bn provision for claims in the US relating to mortgage-backed securities. RBS's loss will be the eight consecutive year the bank has made a loss.
It is also widely expected that RBS will confirm that it has cut its bonus pool to the lowest level since its 2008 bail-out, by-and-large resulting from the slimming down of its investment bank.
In February last year McEwan said the bank would scale back on investment banking operations and focus on UK retail and commercial banking.