Igor Feitoza, a Brazilian-born entrepreneur, left an angry message on his bank’s Facebook page this week.

“I want to see if you guys will pay my overdue bills and my employees as I can’t access my money which last Friday to pay them,” he wrote following a “ridiculous” three hours spent at the bank.

Mr Feitoza is not one of the many frustrated Royal Bank of Scotland customers hit by the latest technology failure this week. He is a client of Commonwealth Bank of Australia, the country’s biggest bank by market capitalisation, which had its reputation as one of the most digitally advanced lenders tarnished by an outage in its payment and online systems late last week.

As well as provoking outbursts on social media, the technology glitches at RBS, which caused some 600,000 payments and direct debits to go missing, and CBA underline how the world’s biggest banks are often failing to get to grips with the growing demands being placed on their IT platforms.

Banks spent about $188bn on IT last year and that figure is expected to grow at close to 5 per cent a year, taking it above $200bn by next year, according to Celent, the research company.

Many banks, such as RBS, are plagued by computer systems that have been built up over several decades through acquisitions and new product launches to form a costly and complex patchwork of systems.

“A lot of these programmes get three-quarters done,” says a senior technology executive at one of the largest US banks. “It’s one of these technical debt problems that builds up over the course of time. If you have one thing happen it might cause the whole thing to fall down.”

The cost of maintaining these often ageing and unwieldy systems eats up three-quarters of banks’ IT spending, according to Celent. That leaves only a quarter to spend on innovations to keep up with the rapidly emerging threat from the many technology groups and start-ups trying to steal market share in areas such as payments.

“For a sector that spends significantly more on technology than most other sectors in the world, it is the least innovative, so there is a paradox here,” says Bill Michaels, head of financial services in Europe at KPMG.

As many banks struggle in the post-financial crisis environment to generate returns above their cost of capital, these spiralling costs and inefficiencies are becoming increasingly unacceptable to both managers and shareholders.

RBS, which last year paid a record fine to regulators for a bigger systems outage in 2012, hoped to solve its problems by replacing its core processing engine at a cost of 750m. But in a recent interview, chief executive Ross McEwanconceded there was still a big job to reduce the number of systems and applications at RBS from more than 3,000.

Asian banks spend more than Europeans or US rivals on IT; their spending is growing faster; and more of their IT money is going on new projects than on maintenance. However, they are hardly glitch-free.

In January, a problem in the system linking accounts at Industrial and Commercial Bank of China, the country’s largest lender, with securities brokerages, disrupted Rmb4.9bn in fund transfers, affecting nearly 55,000 customers at 90 brokerages.

As regulators make ever growing demands on banks to provide them with vast amounts of data covering areas from stress tests to anti-money laundering checks, banks are racing to keep their systems up to speed.

Deutsche Bank insiders blamed its failure in this year’s US stress test on years of under-investment in IT that made it unable to meet US regulators’ demands.

Concern is growing about cyber security after high-profile hacking attacks, such as last year’s theft of data on 76m customers from computer systems at JPMorgan Chase.

Executives say this focus on cyber security is a catalyst for change, pushing banks to simplify and upgrade their IT systems. “Making something secure requires it to be consistent and clean and up to date and well managed,” says the US tech executive.

James O’Neill, senior analyst at Celent, predicts that within a decade most big banks will have switched from using costly mainframe computers for overnight processing of customer data to using much more flexible cloud-based services.

If nothing else, the shortage of developers trained in the Cobol programming language that drives most bank mainframes will force them to make the switch. Australia’s CBA has moved to a cloud-based system, while Deutsche recently outsourced many of its applications to a cloud provider.

“The story about legacy systems impeding innovation is a bit oversold,” says Mr O’Neill. “I’m not saying they will go on for ever, but they have done a pretty decent job of supplying new products and services.”

Like many sectors, however, banks are realising the need to harness the power of “big data” to offer better digital services to their customers.

Mr Michael at KPMG says his banking clients know the stakes are high. “They have seen what technology has done to music and home shopping and so far they have been relatively slow to adapt — but the longer they wait the harder it will get.”