Royal Bank of Scotland has served notice with the Office of Fair Trading it has an interest in bidding for NatWest.
RBS says the announcement should not be interpreted as evidence it will definitely make an offer.
The move is not expected to affect Bank of Scotland's attempts to win clearance for its £21bn bid for the beleaguered NatWest group.
The initial deadline for the OFT ruling on BoS is on Friday.
But now RBS has formerly entered the race, the OFT can extend its Friday deadline by a further 15 days.