Taxpayer-backed RBS plans to cut 40 percent of permanent IT staff, or 650 jobs, and 65 percent of contractors which equate to 230 roles, Unite said in a statement.
Unite said there would be 950 full-time IT staff by 2020, compared with 2,200 in 2016, but a spokeswoman for RBS said it had not yet begun any formal consultations on job cuts and no numbers had been set.
"As we develop long term plans for our technology business, we have in the interests of transparency, started to share our emerging proposals on a future operating model with Unite. We have not consulted on any headcount reduction, instead sharing a direction of travel with Unite which is subject to change," the RBS spokeswoman said.
"Our proposed plans are designed to reduce the number of contractors we employ and strengthen our permanent workforce and while we are downsizing in London we are reinvesting in other UK hubs," she added.
RBS, which is more than 70 percent state-owned, is in the midst of a major restructuring aimed at returning the bank to profit after almost a decade of losses. In its 2016 annual report the bank revealed its global workforce has shrunk from around 226,000 in 2007 to around 77,000.
Earlier this month, the Edinburgh-based lender, rescued in a 45.5 billion pound ($59.8 billion) bailout at the height of the financial crisis, beat first-half profit forecasts, showing signs a long-promised recovery is finally gathering momentum.
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