Spain's Santander has confirmed that it has entered a bid for about 300 high street branches being sold by the Royal Bank of Scotland.The banking giant didn't comment on any financial details, but market speculation earlier this week suggest a deal could be worth about £1.8bn.Santander, which is rumoured to be the only bidder, said it is not possible to say when the tender process will conclude.Santander has been snapping up UK assets in recent years, having bought Abbey, Alliance & Leicester and the savings arm of Bradford & Bingley. A deal for RBS's old Williams & Glynn businesses would make it a major player in the UK bank market with 14% of the mortgage market and 10% of retail savings. The group is also keen to expand outside of its core Spanish market, which has collapsed due to the fall-out of the credit crunch and a property market melt-down. It reduced its exposure to Spain in 2007, helping it to ride out the storm, and has also been buoyed by its extensive business in Latin America. RBS, 84% controlled by the taxpayer, has until 2013 to sell the branches, which it has to meet to EU rules on state aid.