The Unite union has said Royal Bank of Scotland (RBS) is planning 900 technology job cuts at its London office by 2020 to reduce costs.
It alleged that RBS intends to cut 40% of its permanent IT staff, or 650 jobs, as well as 230 contractors.
The bank said no individual job was at risk and no figures had been finalised.
An RBS spokesperson said: "We have not consulted on any headcount reduction, instead sharing a direction of travel with Unite which is subject to change."
Rob MacGregor, Unite national officer, said: "Royal Bank of Scotland is continuing with its savage jobs culling program with today's announcement of a 40% in IT staff, totalling nearly 900 staff.
"The decade of slashing jobs has done nothing to boost morale, increase consumer confidence or improve the bank's performance."
RBS, which is 73% owned by the government, has been restructuring ever since it was bailed out in the financial crisis.
Its global workforce has shrunk from 226,000 in 2007 to about 77,000. It has not made a full-year profit in a decade.
In May, it announced it was shedding nearly 250 IT posts as part of an overhaul of its back-office operations.
In London in 2016, RBS employed 2,200 full-time and contract IT staff. By 2020, Unite claims, there will be just 950 full-time staff.
Simpler, smaller bank
RBS said in a statement: "Inevitably as RBS becomes a simpler, smaller bank focused on the UK and Ireland, our technology function will undergo reorganisation and will reduce over time.
"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."
RBS has a bad track record with IT, suffering problems as recently as April. On the day it announced its first quarterly profit since 2015, its subsidiary NatWest was beset by complaints about glitches in its online banking system.
In June 2015, hundreds of thousands of payments failed to reach the accounts of RBS customers.
In 2012, more than six million customers had their wages, payments and other transactions disrupted when a software update was corrupted. The bank was fined £56m by the Financial Conduct Authority.
Mr MacGregor said: "Unite is angry that the massive scale of IT job losses will sap morale, productivity and faith in the company.
"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,"