Bank customers are facing more frustration after the IT fiasco affecting NatWest, RBS and Ulster Bank dragged into a sixth day.

Parent company RBS said it was still experiencing technical issues which meant some account balances may not be showing correctly, while it also admitted some online services were unavailable.

As it battles to deal with a backlog of payments, around 1,200 branches were open on a Sunday for the first time ever. Trading hours will also be extended to between 8am and 7pm on Monday.

The group told customers: "We are still experiencing technical issues with our service which means account balances may not be displaying correctly. Some of our online services such as payments, may also be unavailable. We would like to apologise to everyone who is affected by this."

Stephen Hester, chief executive of NatWest owner RBS, issued a public apology for the chaos and conceded the bank had let down its customers. Around 7,000 staff are on duty in branches but social media sites such as Twitter continued to be flooded with angry messages from customers who found themselves without access to cash over the weekend or unable to pay bills.

The problem with computer software started on Tuesday night and while RBS says it has been fixed, customers were still experiencing problems today.

Mr Hester attempted to reassure customers following mounting fears that thousands of people could be hit with penalty charges if their regular payments - including mortgages - were affected.

"I am very sorry for the difficulties people are experiencing," he said. "Our customers rely on us day in and day out to get things right, and on this occasion we have let them down. This should not have happened.

"Right now my top priority, and the priority of the entire RBS Group, is to fix these problems and put things right for our customers. This is taking time, but I want to reassure people that we are working around the clock to resolve these problems as quickly as we are able."

NatWest has more than 7.5 million personal banking customers but it is unclear how many have been affected. The issues extend to users of Royal Bank of Scotland (RBS) and Ulster Bank, which like NatWest are part of RBS Group. Ulster Bank said about 100,000 of its customers experienced difficulties.