The group, which remains 72% taxpayer-owned after its rescue during the financial crisis, said 62 locations in Scotland under its RBS brand would shut, as well as 197 NatWest sites in England and Wales. In England and Wales the number of NatWest branches will fall to 655.
"It shows the depth of RBS's lack of understanding of island circumstances that their email announcing the branch closure stated there are still "a number of free to use cash machines in your area", before listing cash machines some 27 miles away in South Uist, which people in Barra would have to take a ferry to use".
There will be a total of around 680 redundancies, RBS said.
It is the latest in a slew of banks to announce closures, following revelations from Halifax and Lloyds Bank earlier this week that they would close branches in Norwich, Acle, Bungay, Halesworth and Southwold - resulting in 99 job losses - branch closures and job cuts at Yorkshire Building Society and the possible loss of East Anglia's last remaining Norwich and Peterborough Building Society branches in Norwich and Ipswich.
An RBS spokesman said: "More and more of our customers are choosing to do their everyday banking online or on mobile".
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"Over five million customers now use our mobile banking app and one in five only bank with us digitally".
He added: "The Royal Bank of Scotland has made a decision to decimate its bank branch network".
Unite's national office Rob MacGregor said: "Now serious questions need to be asked about whether these closures mark the end of branch network banking".
"Why is the Government signing off this alarming branch closure programme?"
"We provide a range of alternative ways to bank, tailored to the needs of our customers and reflective of the way they live their lives".
West Lancashire MP Rosie Cooper commented: "This is terrible and very disappointing news for Skelmersdale residents and for the staff at the branch whose future may be unclear at this point".