The Royal Bank of Scotland is planning to show 40 per cent of its London-based IT staff the door by 2020, according to Unite the union.
The mostly taxpayer-owned bank is also preparing to cut contractor numbers by 65 per cent, the union said.
The announcement would mean around 650 permanent staff and 230 contractors could lose their jobs, and is the latest in a line of cuts at the bank.
Some 600 jobs were lost in 2016, the union said, while there were reports earlier this year that 92 techies' jobs would be outsourced to India.
"Royal Bank of Scotland is continuing with its savage jobs culling program," said Rob MacGregor, Unite national officer.
"By 2020 just a fraction of the RBS IT function will remain, leaving this organisation operating a skeleton service with the customers and remaining staff paying the price."
However, in its own canned statement, RBS said that, as it becomes a "simpler, smaller bank" its technology function "will undergo reorganisation and will reduce over time".
RBS said it had "started to share emerging proposals on a future operating model with Unite" but that it had not consulted on any headcount reduction.
"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."
But Unite countered that the cuts would affect consumer confidence, undermine the company's ability to function properly, and affect staff morale.
"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," MacGregor said.
He added that, as the bank is funded by British taxpayers (the government owns a 71 per cent stake), it "should be concentrating on investing in jobs here in the UK, rather than wholesale cuts".
The bank has regularly reported losses, and the government was earlier this year forced to acknowledge that shares in the bank might be sold at a loss.
Chancellor Philip Hammond told MPs in April that the government was not actively marketing its stake in the bank, adding: "We must live in the real world and make decisions on the future of our holding in RBS in the best interest of taxpayers."
RBS has also suffered major IT problems. Back in 2012, it was reported that an "inexperienced operative" at the company had crippled its ability to process payments for customers of RBS, NatWest and Ulster Bank.
Then-CEO Stephen Hester said there was "no evidence" that this was connected to the outsourcing of IT roles to India, but El Reg noted that the group had been hiring Indian techies to work on the system that failed.
RBS would not be alone in cutting its UK-based operations. HSBC said in May that it will cut more than 800 UK-based tech roles as part of a wider redundancy programme to wipe out 8,000 positions in the UK and 50,000 worldwide.
And in June, The Reg revealed Lloyds Bank had signed a 10-year outsourcing deal with IBM that would see around 500 staff transfer to IBM on September 1, 2017. However, previous warnings from the bank's union had said that after four years the work would be off-shored. ®