Royal Bank of Scotland Group PLC said Monday that it aimed to raise as much as $3.5 billion by floating a quarter of its holdings in its U.S. unit Citizens Financial Group Inc, in what could prove one of the largest U.S. bank listings since the financial crisis.

RBS, which is 80% U.K.-government owned, said in a regulatory filing that it would sell 140 million shares in its U.S. retail bank for between $23 and $25 a share. RBS has also granted a 30-day overallotment option of up to an additional 21 million shares. At the top of the valuation range the deal could value Citizens at $14 billion.

Selling shares in the bank is a key plank in RBS's plan to bolster its finances as it comes under pressure from British regulators and politicians to refocus on its U.K. operations.

"The planned divestment will significantly improve RBS's capital foundation," RBS Chief Executive Ross McEwan said in written remarks. RBS had previously pledged to sell a stake in the bank by the end of 2014. The bank's executives have said they expect to fully exit Citizens by the end of 2016.

Citizens, based in Providence, R.I., operates one of the largest regional banks in the U.S., with $122.2 billion in assets, 18,600 workers and 1,370 branches across the New England, mid-Atlantic and Midwest regions.