On the day it posted higher quarterly results, RBS said it would sell an 80 percent stake in its Global Merchant Services business, which runs the WorldPay payment processing company, to Advent International and Bain Capital for an enterprise value of up to 2.03 billion pounds ($3.23 billion).

Advent and Bain have been in exclusive talks to buy WorldPay since late July, and analysts had given a price tag of between 2-2.5 billion pounds for WorldPay.

The private equity companies said they planned to grow the WorldPay business and could make further acquisitions.

RBS Chief Executive Stephen Hester has embarked on a wide-ranging asset sale program after the bank, 83 percent state-owned, was ordered last year by European regulators to sell a string of assets as a price for its state bailout.

Earlier this week, it sealed the sale of over 300 UK branches to Spanish rival Santander for 1.65 billion pounds.

RBS will keep a stake of some 20 percent in WorldPay. The sale will result in a gain of around 850 million pounds and boost its core Tier 1 capital ratio by around 30 basis points.

RBS will get an initial cash payment of 1.7 billion pounds once the transaction is finalized and could get a further 200 million pounds depending on WorldPay's performance.

RBS shares closed down 1.7 percent at 51.10 pence, giving the bank a market capitalization of around 29 billion pounds.

The stock has risen around 75 percent since the start of 2010, outperforming a 45 percent gain in rival part-nationalized UK bank Lloyds.

RBS had to be rescued by the British government in October 2008 after its finances were stretched by its shared acquisition of Dutch bank ABN Amro in 2007.

The bank was propped up with 20 billion pounds of taxpayers' money, causing the eventual resignation of then chief executive Sir Fred Goodwin, who had presided over an aggressive acquisition policy which included the takeover of NatWest.