The Spanish bank has walked away from from the £1.65bn deal despite two years of negotiations following its agreement to purchase the unit in August 2010.

RBS – which was ordered by the European Commission to sell the branches because it receives state-aid – now has 15 months to find a new buyer.

Billionaire Richard Branson’s Virgin Money is understood to be plotting a new offer, having made a preliminary approach on Friday.

Others, including NBNK, are also reported to be interested.