"I understand that next week they will announce a massive rights issue, that's a demand for new cash from their shareholders," said BBC business editor Robert Peston.
Our BBC correspondent has also learned that RBS will make about £5bn of write-downs when it launches its rights issue next week, a record so far for a British bank.
"This may sound like horrible news," he said but he added: "investors may well see it as a recognition that RBS has properly owned up to its past boo-boos - and they would view such catharsis as very good news".
The rights issue is thought to be a prudent measure to provide a capital cushion for the amount of risk on its balance sheet after RBS played a leading role in last year's takeover of the Dutch bank ABN Amro.
WHAT IS A RIGHTS ISSUE?
Companies issue extra shares to raise money
They are offered to existing shareholders, usually at a discount to the current share price
Shares are offered in proportion to existing holdings, so if you own 10% of the old shares you are offered 10% of the new ones
"Having spent the best part of 70bn euros ($111bn; £56bn) last year on the biggest bank in Holland, they now have the smallest cushion relative to risk on their balance sheet of any bank in Europe," said Alex Potter, banking analyst at Collins Stewart.
Some analysts say this will anger big investors who will see it as an embarrassing U-turn from chief executive Sir Fred Goodwin's former stance.
"It is going to be pretty humiliating for the management," Simon Maughan at MF Global Securities.
"They went out there and did a big acquisition, saying they didn't have to raise equity, and now they are saying that they do. That is pretty embarrassing and I think that institutional investors will demand that heads should roll," he added.
Analysts stressed the rights issue was not something that should worry people with accounts at any RBS banks.
"This is not a customer issue, it's a shareholder issue," said Justin Urquhart Stewart from Seven Investment Management.
Heads of many of Britain's biggest banks, including RBS, had a meeting at Downing Street on Tuesday to discuss the continuing effects of the credit crisis.
According to the BBC's Robert Peston, the Bank of England will announce a plan next week to give banks access to £50bn in government bonds in exchange for UK mortgage-backed securities to encourage banks to lend to each other again.
This in turn should ease up lending to individual borrowers.
Adam Shaw explains what a rights issue involves
Liberal Democrat treasury spokesman Vince Cable supported the idea of a rights issue in addition to the Bank of England's proposals.
"It's positive and it's necessary and it's got to happen for all of the big banks," he told the BBC.
"Essentially this is all about sharing pain - shareholders have got to accept that any losses arising from the credit crunch accrue to banks and not to the taxpayer."
The government and the Conservatives declined to comment.
Analysts expect other banks to follow RBS by going to the market for extra capital.
Bradford and Bingley denied widespread reports at the weekend that it was planning to raise money in the same way.
RBS shares have had a volatile day's trading, opening higher before falling and then rising again. They rose almost 5% and closed at 384 pence at the end of London trade.
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