Thousands of workers are facing mounting uncertainty over their jobs after the collapse of a proposed sale of Royal Bank of Scotland branches to Spanish banking giant Santander, it warned.
Unite called on the government to press the European Commission to lift its requirement for RBS to sell 316 branches and other assets.
ITV News's Ellie Price reports
The UK state-owned bank had planned to sell the assets, which also included 40 banking centres for small and medium-sized businesses, in a deal approved by the EC.
But Santander has pulled out of the £1.65 billion deal, sparking fresh job fears.
Gail Cartmail, Unite's assistant general secretary, said:
RBS Group chief executive Stephen Hester promised there would be "no disruption" for customers and said the taxpayer-backed bank would begin a process to seek a new buyer.
Santander agreed in August 2010 to buy the assets, broadly comprising the RBS branch business in England and Wales, the NatWest branch business in Scotland and certain other business activities across the UK.
But last night it said it had pulled out of the deal after it became apparent that a revised target for the purchase to be completed by the end of the year would not be achieved.
The Spanish bank said the sale was originally scheduled to be completed in 2011 but this was extended to the end of 2012.
Santander UK chief executive Ana Botin said:
Some commentators have suggested that finding another buyer in the current economic climate may not be easy.