American billionaire Wilbur Ross has pledged half a billion pounds to help Virgin Money bid for Royal Bank of Scotland's network of branches.
Over the weekend, Mr Ross agreed a deal to buy 21% of Sir Richard Branson's Virgin Money for Stg 100 million.
The promise to provide another Stg 500 million comes ahead of a deadline for bids for the bank's chain of 300 branches.
The BBC reports Virgin is expected to make a formal announcement on Tuesday that it is teaming up with Mr Ross.
Mr Ross previously teamed with Virgin in 2007 in an attempt to buy Northern Rock, which was also bailed out by the British government.
Other possible bidders include Spanish bank Santander and National Australia Bank.
Virgin Money declined to comment.