Image caption The problem prompted apologies and complaints on social media

Banks have "creaking" computer systems but the payments failure at RBS was "unacceptable", the head of the UK banking body has said.

Anthony Browne, chief executive of the British Bankers' Association (BBA), said banks were spending £3bn a year upgrading systems.

About 600,000 transactions were delayed as a result of the processing problem at the RBS group on Wednesday.

The bank said that the backlog would be cleared by the end of Saturday.

Digital banking

Payments of wages, tax credits and disability living allowance were among those that failed to be credited to accounts of customers at RBS, NatWest, Ulster Bank and private bank Coutts.

The underlying problem was fixed on Wednesday morning but many of those affected are angry that it will take until the end of the week for all payments to go through.

Mr Browne, of the BBA, told the BBC the incident was unacceptable, repeating RBS's admission on Wednesday.

"The banks are already aware of this as an issue," Mr Browne said.

"A lot of them have creaking IT systems. They are spending £3bn a year upgrading their IT systems - it is one of the biggest IT upgrades that Britain has ever seen."

Most banks and building societies regularly suffer from online banking glitches, such as a problem at Nationwide earlier in the week, despite promoting the increased use of digital banking.

During a presentation on banking technology on Thursday, Simon McNamara, of RBS, said it was not feasible for a system to be faultless for 100% of the time.

"Technology will, on occasion, fail. If and when that occurs, we need to ensure we can mask the impact on customers and recover as quickly and effectively as possible," he said.

"It is important that it is handled well and competently."