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This week's computer chaos at TSB is unlikely to be the last time millions of people face online banking problems.

In recent years computer systems at NatWest, RBS, HSBC, Ulster Bank have all gone into major meltdowns.

Now IT experts warn that legacy systems and quick fixes have left most main banks at risk of outages.

Former banker Philip Augar told the BBC: "Bank systems have got sticking plaster all over them".

Speaking on the BBC's Today programme, Mr Augar, a former non-executive director at TSB, warned: "There's need for an absolutely radical overhaul. They all have to do it."

Complex systems

With millions of us now reliant on online banking, it appears that consumers need to have a back-up plan – for example, by opening up a second bank account – to be able to cope with any future problems.

That's especially true as banks have been busy closing thousands of branches, leaving many of us with no local bank manager to turn to.

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Nick Hammond, ex-global head of networks at Barclays and now lead adviser for financial services at World Wide Technology, said: "The crisis at TSB provides a great case study for the problems presented by the sheer complexity of many banking IT systems, and especially, the difficulty involved in changing them."

The IT systems at traditional banks have grown into "complex architectures that have organically grown after decades of successive development and component re-platforming", he explained.

It means that carrying out any kind of upgrade or transition can be extremely difficult.

Lev Lesokhin, an analyst at CAST, a company that measures and analyses software to help banks, pointed out: "Complex IT systems are very unstable when upgraded.

"Banks should identify software issues and agree on a migration plan before they undergo digital transformation projects, otherwise, their customers will suffer."

New rules

But it's not just upgrades and customer migrations that will lead to problems in the future.

Computer systems will have to be adapted to take account of new rules, warned Mr Hammond.

For instance, the General Data Protection Regulation (GDPR), which forces firms to protect their customers' information, comes into force on 25 May this year.

"Regulations coming into force this year require huge technical changes which may lead to similar problems for many established banks," he said.