UK banks need to raise billions more in capital to cover their risks, according to the financial regulator.
The Prudential Regulation Authority (PRA) says Britain's top banks and building societies need to fill a £27.1bn hole in their balance sheets.
Royal Bank of Scotland was the regulator's main cause of concern, accounting for £13.6bn of the total.
Lloyds Banking Group accounted for £8.6bn and Barclays £3bn. Nationwide had a small shortfall of £400m.
Co-operative Bank has already identified a £1.5bn hole in its finances and announced a bond-to-equity 'bail-in' plan to deal with the shortfall.
HSBC, Santander UK and Standard Chartered were given a clean bill of health by the regulator.
The Bank of England's Financial Policy Committee had asked the PRA to review the state of UK banks' financial health against new guidelines agreed by international central banks.
These guidelines - known as Basel III - require banks to hold capital resources of at least 7% of their "risk-weighted assets".
The PRA concluded that, as at the end of 2012, Barclays, Co-op, Lloyds, Nationwide and RBS "fell short of this standard".
Since then the banks have taken action to start to fill the gap to the tune of £13.7bn, says the PRA, although some of their proposed measures still need regulatory approval.
Even after these actions, four out of five still fall short of the 7% standard, warns the PRA, and are required to submit further plans detailing how they will plug the £13.4bn hole that still remains.
The PRA expects most of the banks' remedies, which include restructurings and sell-offs, to be implemented by the end of 2013.
Responding to the report, Lloyds said the bank had a "strong capital position" and that its capital adequacy ratio - known in the jargon as Basel III Core Tier 1 capital ratio - would be more than 9% by the end of June, and about 10% by the end of the year.
RBS said it "continues to target" a figure of 9% by the end of 2013, while Barclays feels confident it can plug its £3bn gap through disposals.