Royal Bank of Scotland has been warned that it faced a potential downgrade by the credit rating agency Moody's which is concerned that last month's profits warning may have weakened its financial strength.

The 81%-taxpayer owned bank is expected to report losses of £8bn in a fortnight's time following its admission it would incur an extra £3bn of losses for the US sub-prime mortgage crisis and mis-selling of payment protection insurance and interest rate swaps.

The extra cost forced Moody's to rethink its assessment of the bank's strategy and said its "management faces a number of short-term headwinds, which could challenge the implementation of this plan and in turn be negative for its creditors".

"In addition, Moody's believes that the overall downside risks associated with the bank's recovery have increased," the agency said.

The possibility of a downgrade for the bank's A3 rating comes ahead of the publication of its results on 27 February when its new boss Ross McEwan, below, is also scheduled to announce his strategy for the bank after taking the helm on 1 October, including cutting costs and scaling back on some business lines.

Moody's announced its concerns about the bank shortly after McEwan had taken questions from readers on the Guardian's website in which he defended the need to pay bonuses d'espite the losses, promised to put business bankers into 300 branches, and admitted that a computer meltdown two years ago showed that the bank needed new IT systems. He said the bank would spend whatever it took to make the systems more reliable.

He also countered criticism, such as that raised by Lawrence Tomlinson, an adviser to Vince Cable, that there was systemic fraud inside its restructuring division and admitted that the bank had got off to a "slow start" in dealing with claims about interest rate swap mis-selling.

McEwan defended bonus payouts to bankers: "I'll pay in the market to get the best people and to hold onto them," he said. He pointed out that bonuses were paid out of operating profits not the reported totals which have shown losses since the 2008 banking crisis.

"To attract and retain good people, I need to be paying on or around the market, otherwise I put at jeopardy some of these core businesses," McEwan said.

The amount the bank intends to pay out in bonuses is expected to be announced along with its 2013 figures, though it is expected to be around £500m. The bank is also facing further controversy over how it approaches the EU limit on bonuses to 100% of salary. This cap can rise to 200% if shareholders approve and the bank is expected to ask for such support at its annual meeting in May.

Asked to justify huge payouts to bankers whose roles were not socially useful, McEwan said: "There is a small group of people that have some specialised skills in putting together 'trades on corporate customers', that add really good value to those businesses. It comes down to supply and demand – there are few people that have these skills (again globally), and we need to pay the market rate to attract these people to our organisation, to perform their roles on behalf of large corporates.

"I think we would be in danger of losing some of our most skilled investment bankers if we didn't pay the market rates. But let me be quite clear, it needs to be based on performance. When performance is up, then rates can be up. When performance is down, rates will go down," he said.

The IT meltdown in June 2012 which left customers unable to access their accounts for weeks and for as long as a month in the Republic of Ireland and Northern Ireland. It showed the bank needed to upgrade its system and hire the right people. "The incident showed we'd lost a lot of core skills on how these very complicated old systems operated," McEwan said.

The bank is also considering closing up to 36,000 non-sterling accounts, according to Sky News, because of concerns about money laundering and a determination to cut costs by McEwan.