Royal Bank of Scotland will have to raise around £2 billion to boost its financial strength after failing the Bank of England's annual health check of the sector.
The lender, which is 73% owned by the taxpayer, emerged as the worst performer in the stress test and has drawn up a plan overnight to bolster its resilience in case of a financial crisis.
Barclays and Asian-focused player Standard Chartered also struggled in the test, but the Bank said their existing plans mean they do not have to take further action.
The Bank's most severe annual stress test so far gauged the resilience of seven UK lending giants - RBS, Lloyds Banking Group, HSBC, Barclays, Santander, Standard Chartered and Nationwide Building Society - against a global economic crisis and crashing house prices.
The Bank's Financial Policy Committee said in light of the findings and action taken by RBS, "the banking system is in aggregate capitalised to support the real economy in a severe, broad and synchronised stress scenario".