Britain's beleaguered banks must quickly inject billions of pounds into their creaking IT systems, on top of forking out for the many scandals that will continue to hit banking results this week.
The warning comes in the wake of computer meltdowns at NatWest and Nationwide that caused chaos for customers and further damaged the reputation of the industry.
This week, two of the biggest players in the sector, HSBC and Royal Bank of Scotland, will reveal another series of write-offs in their half-year results.
HSBC Mexico is under investigation for alleged non-compliance with money laundering regulations, and the parent bank is among those being scrutinised by the Libor-rigging inquiries. These costs are on top of the money set aside for the mis-selling of personal protection insurance (PPI) to individuals and interest-rate hedging products to small and medium sized businesses.
RBS, the owner of NatWest, could reveal a loss of up to £100m on the computer melt-down that led to a complete disruption of service in June. The bank, 82 per cent owned by the taxpayer, suffered a collapse of its systems that led to calls for investigations from the Government and the Bank of England.
That loss is a mere slice of what it needs to spend to bring its systems up-to-date, say experts.
Temenos, the software group that provides banking technology for leading banks, including Metro Bank, says the entire sector is running on outdated systems that require massive investment.
Ben Robinson, a director at Temenos, says: "The IT infrastructure of many of the world's leading high-street banks is decades old and is struggling with the rate of change in the industry and data demands that banks are experiencing today.
"Recent well-publicised system problems could have happened at any number of large retail banks, and if action is not taken now such issues are likely to become more common.
Computer experts say that banks have not been properly regulated when it comes to IT systems and that they have failed to invest properly as they sought to maximise profits in the boom years.
Mr Robinson added: "It's not surprising that the subject of legacy systems, in addition to dragging down bank profitability and becoming more of a focus for investors, is now on the regulators' radar."
Last week, Lloyds Banking Group made a first-half loss after increasing the money it set aside for PPI mis-selling claims by £700m. Barclays' PPI bill stands at £1.3bn.