While RBS confirmed its review of Ulster Bank was still ongoing, it would not be drawn on suggestions KKR and Apollo are among the front runners to buy the parts of the subsidiary held in the Republic of Ireland.
The current management team at Ulster Bank, led by chief executive Jim Brown, is also believed likely to take some sort of stake if a deal can be struck.
The parts of the bank held in Northern Ireland are not thought to be included within the discussions and would probably be integrated into the Natwest network should the separation go through.
A price tag of between £395 million and £2 billion has been mooted but is dependent on which assets are sold.
Any transaction will hinge on RBS taking on some of Ulster Bank's distressed assets and providing the purchasers with some form of indemnity against future losses.
With RBS still majority owned by the British government that structure of deal may prove difficult.
Morgan Stanley has been working with RBS in assessing its options for Ulster Bank.
It is the biggest bank in Northern Ireland and third biggest in the Republic.