One of the bidders has warned that the sale, known as Project Rainbow, could drag on until 2016, according to the Daily Mail.
W&G Investments, made up of 25 blue-chip UK investors, is proposing to buy the branches for an initial £1.1bn.
It said: “It is possible that separation will not be achieved within the two-year period currently contemplated.”
W&G also said that it was setting a four-year deadline – after which RBS would have to repay any money W&G had already put down.
Three parties have submitted bids to take on the branches, with a preferred bidder likely to be announced in September.
The European Commission made it a condition of the government’s bailout of the group that it sell the branches by December of this year. RBS is set to ask for an extension.
