The bank, which is still 71 per cent owned by the taxpayer, will cut 650 jobs by 2020, the Unite Union has claimed.
In 2016 RBS employed 2,200 IT staff but by 2020 there will be just 950 full time staff, according to union and FTAdviser's sister paper, the Financial Times.
The bank was fined £58m in 2014 after IT failures left customers without account access, while it suffered another outage in 2015. The company announced sweeping cost cutting plans earlier this year, aiming to strip out operating expenses of £2bn by 2020.
The company has also moved some staff jobs offshore.
Rob MacGregor, Unite national officer, said: “RBS’s fixation with cutting employee numbers, restructuring and offshoring work that could reasonably be done by displaced staff within the RBS IT community is unacceptable.
"This British-taxpayer funded bank should be concentrating on investing in jobs here in the UK, rather than wholesale cuts.”
Unite has said the bank is also cutting more than 200 IT contractors.
HSBC has also announced that it will be cutting 840 IT roles in the UK and creating jobs in India, China and Poland instead.
In a statement RBS said this was a "direction of travel" which is subject to change and it had not consulted on any headcount reduction but it had shared its plans with Unite "in the interests of transparency".