Troubled TSB is braced for two tough days as customers continue to face difficulties following the bank's IT switchover.
The bank admitted some customers had problems with standing orders on Tuesday - the first day of the month, popular for regular payments.
It had previously said these were working normally, but there could be problems with some other payments.
On Wednesday, the bank's bosses face questions from Treasury Committee MPs.
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TSB is urging people who use internet banking to log in via the official website, rather than via old bookmarks or links. Those having difficulties using the mobile app are being told to reinstall the app and try again.
Some customers have complained that payments have failed to complete, with some raising concerns over standing orders.
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"We know there are some issues with a small number of standing orders today, we are working hard to resolve these," a TSB spokeswoman said.
Problems began after TSB shut down services for two days from the evening of Friday, 20 April to move customer data from former owner Lloyds to a new IT system managed by its Spanish owner Sabadell.
As soon as the new system was switched on, customers reported problems which have persisted for some banking online and in branches.
On Wednesday, senior executives from TSB and Sabadell are appearing before the Commons' Treasury Committee.
They will face questions about the cause of the IT crisis, the information - at times contradictory - given to customers, and the plans to compensate those affected.
TSB chief executive Paul Pester will be giving evidence, alongside TSB chairman Richard Meddings, and Miquel Montes, the group chief operating officer at Sabadell.