NatWest will open branches this Sunday as the bank deals with the ongoing chaos caused to customers who have been hit by technical problems.

Angry customers have vented their frustration about the hitches, which are also affecting Royal Bank of Scotland (RBS) and Ulster Bank users, with some finding that they had supermarket deliveries stopped and others having disappearing wages or holidays and home purchases disrupted.

All three banks are part of RBS Group, which said it is clearing a backlog of problems caused by a three-day technical issue which it believes is now fixed.It has apologised and said it is "working round the clock" to correct the problems, including extending its opening hours.

A group spokesman said: "Over 1,000 of our branches will be open until 7pm tonight to help customers with inquiries or who need access to their cash. In addition, our branches open on Saturday will extend working hours to 6pm, and these branches will also be open this Sunday from 9am to 12pm."

Account balances have not been updated properly overnight, meaning that credit and debit payments are not showing up as quickly as they should, although RBS Group said the money is "in the system". People going into their branch could not necessarily see the most up-to-date information on their balances, although staff were said to be "geared up" to help.

Fears have been raised that thousands of customers could be hit with penalty charges if their regular household bill payments such as mortgage payments are affected. The banking group has promised that no one will be out of pocket and anyone who incurs such a penalty as a result of the problems should contact their branch.

Susan Allen, customer services director for RBS-Natwest retail, said it was difficult to say exactly when all the problems will be resolved. She said they were due to "an error in our system which we believe we have now fixed but we are clearing the backlog".

NatWest has more than 7.5 million personal banking customers, but the group said it could not tell how many have been affected as it was not possible to know when they were expecting payments into their accounts.

However, Ulster Bank said that around 100,000 of its customers have been affected by the same issues. The problem reportedly arose after staff tried to install a software update on RBS's payment processing system, but ended up corrupting it.

With the bank's advertising slogan Helpful Banking attracting the attention of many tweeters, there were numerous messages on Twitter about wages not appearing in accounts or the prospect of being hit with late payment charges.