LONDON (The Deal) -- British government-controlled Royal Bank of Scotland Group (RBS) took the first step toward the long-awaited initial public offering of its main U.S. banking operation on Monday, with the registration of an intention to list Citizens Financial Group Inc.

The move could raise as much as $3 billion for Royal Bank as the selling shareholder, according to an estimate by Renaissance Capital LLC analysts, in a report carried on the Nasdaq website. In a Securities and Exchange Commission filing on Monday, Citizens said the initial offering would be of up to $100 million, including stock placed in a potential overallocation option, but made clear this was an estimate made "solely for the purpose of computing the amount of the registration fee."

The filing said Royal Bank of Scotland had appointed Morgan Stanley and Goldman, Sachs & Co. as joint global coordinators and JPMorgan & Co. as joint bookrunner. Citizens' chief legal officer Sheldon Goldfarb turned to Davis Polk & Wardwell LLP and Cleary Gottlieb Steen & Hamilton LLP for external legal advice.

The filing came as Royal Bank continues with its post-crash strategy of shrinking its overseas presence, winding down its investment banking operations and focusing on the U.K. retail and commercial lending markets. Since its 45.5 billion pound ($76.7 billion) taxpayer bailout, the bank has been under political and regulatory pressure to reduce market share, sell off branches and operations and sharply increase its regulatory capital.

The S1 filing contained little information about Royal Bank's ambitions for the offering. It did not specify the exchange where the shares would be listed and gave no timetable for a flotation other than to say it would be "as soon as practicable."

It said that Citizens itself would raise no new funds in the IPO and that Royal Bank would remain the controlling shareholder for the time being. Pointing out that Edinburgh, Scotland-based Royal Bank is 79.9% owned by the U.K. taxpayer, it said the bank plans to sell further holdings, but the timing was uncertain.

In a stark reminder that acquirers of any stock do so at their own risk, the prospectus warned that Royal Bank could theoretically choose to sell its controlling stake to a strategic rival or a single financial investor.

"If RBS sells a controlling interest in our company to a third party in a private transaction," it said, "you may not realize any change-of-control premium on shares of our common stock and we may become subject to the control of a presently unknown third party."

A sale before or after any potential IPO cannot be ruled out. Earlier this year, Royal Bank had to play down media reports that it was in exclusive talks about selling the business to Sumitomo Mitsui Financial Group Inc. and that Mitsubishi UFJ Financial Group Inc. had considered a bid.

"We announced in November 2013 that we would accelerate the [initial public offering] of Citizens, aiming to start the process in the second half of 2014 and to fully sell the business by the end of 2016. That is our plan," the U.K. bank said in an e-mailed statement, at the time.

Prior to November, however, the bank had been less ready to make such a categorical commitment. Former CEO Stephen Hester, who was forced to resign by the government, had been disinclined to sell the U.S. operation at all and only reluctantly agreed to an IPO. Even his successor, Ross McEwan, had previously indicated that while the bank was planning an IPO, it would still be open to approaches from potential buyers.

Citizens, of Providence, R.I., is the 13th-largest retail bank in the U.S., with $122.2 billion of total assets as of Dec. 31. At the same date, including assets and liabilities classified as held for sale, it had loans of $87.1 billion, deposits of $92.2 billion and stockholders' equity of $19.2 billion. Citizens generated revenue of $4.7 billion in 2013.

The filing added that the bank operates through two segments, consumer banking and commercial banking, which last year contributed 55% and 45% respectively to Citizens' loan book.