One of the biggest transfers of banking data ever attempted in the UK, involving the switch of 1.3bn TSB customer records, fell into turmoil as millions of customers were locked out of their accounts.

Some customers alleged that the IT “upgrade” had left them with rogue credits and debits on their accounts, while others complained they had been given access to random accounts.

TSB said its 1.9 million mobile and internet banking customers suffered “intermittent” failures to access their accounts on Monday, while for about half an hour a glitch gave some users a view of “nominee” accounts they would not normally see.

The transfer had been planned for months, and was supposed to be completed by Sunday evening, but on Monday morning the bank was bombarded with complaints from customers unable to log on.

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 over the weekend it moved all customer accounts to a system designed by Sabadell for its UK customers.

Customers were warned that they would not be able to conduct transactions on the internet or their mobile phone over the weekend. When problems emerged on Monday morning, TSB said it hoped to have its systems “up and running again soon” but the outages persisted all day.

By the end of the working day TSB was still unable to reopen all accounts. It told customers: “We don’t have any timescales that we’re able to provide at the moment. However, we are working to fix it as fast as we can.”

A spokesperson for TSB said if any customer was left out of pocket as a result of the IT transfer they would be reimbursed.However, TSB insisted customers were able to use their cards throughout the period.

It said the planned transfer from 4pm on Friday through to 6pm on Sunday meant customers were not able to make payments into and out of their accounts using the internet or mobile phones, and apologised that for some customers the services remained unavailable on Monday.

“We are really sorry for the inconvenience this is causing our customers and want them to know we are working as hard and as fast as we can to resolve this problem,” TSB said.

In an own goal, however, TSB’s parent group, Sabadell, issued a press release with a statement from the group chair, Josep Oliu, saying it had “successfully completed the TSB technology migration”.

On Twitter, customers shared images of rogue debts and credits on their accounts. One bemused account holder showed his TSB banking app recording a direct debit paid to Sky Digital – in 81 years’ time.

“Now I know there are a lot of theories about time travel, but look I have actually gone 81 years into the future and paid a direct debit to Sky. Just thought you should know!” Ben Stratton tweeted.

Others were more unhappy about the prolonged outage. Paul Scriven, a House of Lords peer, tweeted that his account was wrongly showing a £0 balance. He wrote: “Stop telling me and other customers you have an intermittent problem. You clearly have a major systems failure. Be honest. It’s bad enough for us your customers without you taking us for mugs.”