W&G Investments - a consortium of well-known City names, led by former Tesco finance director Andy Higginson - tabled its formal proposal for the RBS branches after listing its shares on the Alternative Investment Market (AIM) in order to raise an initial £15 million to fund its bid.
It suspended the shares yesterday until the outcome of the auction is known.
If its bid is successful, a raft of fund managers would use the listed company to plough in cash to complete the acquisition of the branches, which are being offloaded by RBS in order to meet European rules on state aid.
It is thought a decision could be made by RBS by the end of the month, when it will choose between the bidders or a possible flotation, which would see it follow in the footsteps of fellow state-backed player Lloyds Banking Group.
W&G - named after the Williams & Glyn's brand being sold by RBS in conjunction with the branches - is one of three potential buyers, alongside Corsair Capital, which is working with US private equity firm Centerbridge Capital, and a joint bid from AnaCap and Blackstone.
W&G is proposing to pay £1.1bn upfront for the branches with a potential £400m due later, depending on the performance of the branches.
The book value of the RBS branch assets is £1.55bn.