An IT meltdown at TSB is causing chaos for millions of customers.The TSB chief executive, Paul Pester, has apologised, but that may not be enough. Amid reports of people losing out on property purchases or being stranded without money on the continent, senior politicians are now demanding answers.

What went wrong?

Problems started to emerge on Monday morning, following a planned IT upgrade scheduled for between 4pm on Friday 20 April and 6pm on Sunday 22 April. Customers had been warned that online banking would be unavailable between these times, as would most mobile banking, while branches and phone banking would offer a limited service, and payments and money transfers would also be affected.

But, hours after the work was supposed to be finished, the bank was bombarded with complaints from customers unable to log on.

Some customers also said the upgrade had left them with rogue credits and debits, while others complained they had been given access to random accounts.

How many customers are affected?

TSB has about 5 million customers, though of that number, about 1.9 million are active mobile and internet banking users. The bank said they suffered “intermittent” failures to access their accounts, while a glitch gave some users a view of “nominee” accounts they would not normally see.

Have they sorted it out yet?

No. TSB said on Tuesday it was “still seeing issues with access to our digital services”. However, the deluge of unhappy customers complaining on Twitter would suggest that that is putting it mildly.

Many remain locked out of their accounts and unable to make payments. However, some customers said they were able to make payments with their debit card and withdraw cash from ATMs as usual.

Nicky Morgan MP, the chair of the Commons Treasury committee, said the incident had “all the hallmarks of an IT meltdown”.

What is TSB doing about all this?

The bank said on Tuesday it was taking its online banking and mobile app down “for a few hours,” adding: “We hope to be back up later this afternoon. We’ll let you know as soon as it’s available again.”

Why was it conducting an upgrade in the first place?

TSB was carved out of Lloyds Bank and then bought in 2015 by Spain’s Banco Sabadell. Initially, Lloyds continued to handle the IT infrastructure for TSB, but this upgrade was about moving 1.3bn customer records over to a “state-of-the-art” banking platform called Proteo4UK, designed by Sabadell.

Will I be compensated?

TSB said: “Customers can rest assured that no one will be left out of pocket as a result of these service issues.”

So that should mean people will be compensated if, for example, they incur late payments fees because they are not able to make a payment. Keep any receipts/paperwork, and visit a branch or call the bank and explain that you want to reclaim your money.

Will TSB be penalised?

It could prove an expensive business for the bank. Following Royal Bank of Scotland’s June 2012 IT meltdown that left some people locked out of their accounts for days on end, the banking group paid out a total of £70.3m compensation to UK customers - including businesses - and £460,000 to individuals and companies who were not customers, but were affected by the problems.

However, RBS is a much bigger bank, and this earlier incident – which did not include a planned upgrade – involved 10% of the UK population: at least 6.5 million customers. In that incident, the disruption suffered by most of those affected lasted for six days, though it was 20 days for large numbers of Ulster Bank customers.

Why do these banking meltdowns keep happening?

You’re right – there have been quite a number in recent years. RBS seems to have more than other banks – it had another big one in December 2013, with customers unable to use their debit cards or log in to internet or mobile banking.

Part of the problem, according to many experts, is that many retail banks have complicated legacy systems that have arguably become unwieldy as a result of being added to and meddled with following mergers and takeovers. The addition of a proliferation of new payment technologies such as contactless payment and Apple Pay, plus the need for all these different systems to “talk” to each other, has added further pressure.

This latest upgrade involved TSB migrating vast amount of customer data from the banking platform it rented from Lloyds to its own platform.