Branson's Virgin Money and Cowdery's Resolution were likely to make first-round expressions of interest in Northern Rock this week to UK Financial Investments, the organisation set up to manage the British state's bank holdings, the sources told Reuters.
"We are in receipt of the information memorandum. My expectation is that we would be submitting something," said a source close to the Virgin camp.
Cowdery has a strong following among institutional investors after his first project made millions from buying and merging closed life insurance funds in Britain.
He has traditionally focussed on insurance, but has said there are consolidation opportunities across financial services.
Resolution was among the bidders for mortgage bank Bradford & Bingley in 2008, prior to its collapse.
Virgin, Resolution and Northern Rock all declined comment.
Northern Rock had to be fully nationalised three years ago after nearly collapsing during the credit crunch.
British Finance Minister George Osborne said last month he aimed to find a buyer for it this year.
The government injected 1.4 billion pounds of taxpayers' money into Northern Rock to prop it up. The bank had liquid assets of 5.9 billion pounds at the end of 2010, and posted a loss of 232 million for that year.
Mutually owned savings firms – known in Britain as building societies -- such as Coventry building society and Yorkshire building society have also expressed interest in Northern Rock.
Coventry and Yorkshire have argued that Northern Rock should return to its incarnation as a mutually owned building society, rather than becoming a privatised bank.
Analysts have also said private equity company JC Flowers may look at Northern Rock.
Virgin Money and Resolution were also among six parties interested in buying 632 bank branches, which part-nationalised lender Lloyds has been forced to sell by regulators.
However, Virgin Money sees Northern Rock as a more attractive proposition than the Lloyds branches due to its better infrastructure, the source close to Virgin said.