Investec has downgraded its rating for UK banking group Royal Bank of Scotland (RBS) from 'hold' to 'sell', reminding investors of the rule: 'Never own RBS into the numbers!'"'Short RBS' may be a crowded trade but after a 19% one-month 'spike', we think it (once again) looks attractive," said Analyst Ian Gordon.He warned that he sees negative risk to his estimates for one-off charges at RBS, including an expected further £1.5bn for mis-selling, £3.8bn in regulatory fines and £2.3bn for restructuring.Meanwhile, the "very weak recovery" in 'underlying' earnings suggests that the bank's target for return on equity exceeding cost of equity may not be achieved before 2018, Gordon said.He believes there are "plenty of 'micro' issues" which have led to an improving investment case for RBS, leaving Investec "more positively disposed" to the stock than others.However, Gordon said: "The 'big picture' of a painfully slow recovery in earnings/returns still constrains our view on valuation. As such, we now recommend reinstating short positions ahead of FY13 results on February 27th." Investec prefers exposure to the remaining banks in the sector, reiterating its 'buy' ratings for Standard Chartered, HSBC, Barclays and Lloyds.RBS was trading down 2.14% at 363.55p before the close of trade.BC