The statement came after the City Takeover Panel publicly rejected an appeal by Bank of Scotland's advisers, Morgan Stanley and CSFB, against an earlier ruling that it would not set a date by which RBS would have to clarify its intentions towards NatWest.
Analysts said the remarks came dangerously close to an admission by BoS that it would raise its offer for NatWest if RBS entered the fray.
The rejection of the appeal is a serious blow to Bank of Scotland, which has been trying for weeks to flush out a formal counterbid from its Scottish rival for NatWest. Bank of Scotland had originally asked the Panel to force RBS to make a clear statement of its intentions within 48 hours of the publication of NatWest's defence document. BoS had argued that the uncertainty over RBS' intentions was creating a false market in NatWest shares.
However, the Panel turned down the request on the grounds that it did not want to be seen to be changing the tactical balance in the bid battle by "requiring RBS to decide its intentions prior to the final date for revision of BOS offer".