Most banking customers are heavily reliant on just one bank or building society to access their money, Which? has found, as concern grows over major system failures blocking people from their cash.
Our exclusive research shows that 77% of people either have just one current account, or all their current accounts with one provider. If that provider experienced problems, they may be unable to get any money.
Some 68% either have just one bank card (debit or credit), or multiple cards with the same scheme (Visa, Mastercard or Amex), meaning a scheme failure, such as Visa's large-scale June 2018 crash, would leave them unable to make card payments.
Almost a third say they never or very rarely carry cash, meaning they lack a fallback option if their card fails in a shop.
Our research in May 2019 found that one in seven people had been unable to use their card in the previous year due to a system outage.
Five months later, Parliament's influential Treasury Committee published a damning report on IT failures in financial services.
It branded the number of outages 'unacceptable' and said that firms' inconsistent recording of failures was 'concerning'.
It had stern words for regulators too, exhorting them not to let individual firms 'set their own tolerance for disruption too high'.
We spoke with a source who has more than 20 years' experience working with systems at banks both large and small.
Speaking on condition of anonymity, he said the immense pace of change meant systems become 'legacy' (at least partially outdated) in as little as five years - yet some traditional banks have 50-year-old systems.
While some legacy systems may still be quite resilient, they're not geared to the demands of modern banking.
Huge IT upgrades or migrations can be one response, but they are staggeringly expensive. And as the TSB debacle shows, when things go wrong the results can be catastrophic.
Our source believes that the costs and risks create an 'anti-change culture' at some banks, meaning they're more likely to keep patching up antiquated systems.
Legacy systems are more of an issue for older and more established banks than their much younger, digital-age counterparts.
Yet when it comes to the risk of cyberattacks, newer banks are just as vulnerable, according to our insider.
Banks of all types are having their systems 'attacked constantly every minute of the day' by 'well-funded bots from China'. In general, he believes most banks do a good job of securing themselves against such threats.
But he criticised the way some banks communicate with customers during outages, with messages having a 'parent-to-child' quality instead of treating customers 'like grown-ups'.
Banks should, he felt, be more willing to explain to customers that they don't know exactly what's gone wrong, but reassure them they're working round the clock to fix it.
When Harriet (not her real name) saw news of the 2018 TSB meltdown, she was very concerned.
Much of her parents' money was in a TSB account, which Harriet - who had Power of Attorney over their affairs - used to pay their care fees.
Alarmed by reports of the chaos, she tried to transfer a chunk of her parents' money to their account at another bank.
TSB had ramped up security monitoring on its accounts amid concerns that its outage had left customers vulnerable to fraud. Harriet's transfer was blocked and the account was immediately suspended.
After being unable to resolve the problem over the phone, Harriet was eventually forced to visit a branch, and waited more than two hours while branch staff themselves tried to reach the call centre and reinstate the account.
They were helpful and apologetic, obtaining £150 in compensation for her after hearing about her experience.
A TSB spokesperson said: 'We're sorry for the issues some of our customers experienced in April 2018, and we have fully compensated them for any loss and inconvenience they faced. Our IT platform is now performing in line with the industry average.'
Nobody should be left out of pocket or suffer a dented credit score as a result of a banking glitch.
The scale of IT issues at banks in recent years may seem alarming, but there's no need to reject modern technology. Failures could also hit the systems that branches run on.
There are practical steps you can follow to protect yourself and mitigate any inconvenience. Ideally, you would carry two cards from different providers, whether they're credit or debit cards.
Registering for various modes of banking, including mobile, telephone and online, can provide you with more options when things go wrong with one.
And cash is still a useful failsafe.
With such a regular rate of IT failures across the banking industry, Which? believes the industry must urgently work to minimise the impact on customers and as a vital backup when systems inevitably fail.
We are calling on the government to introduce legislation that protects cash for as long as it is needed.