A software malfunction is to blame for making RBS and NatWest services unavailable to customers, Computer Weekly understands.
A source close to the situation confirmed it was a software problem that led to two days of downtime at the RBS and NatWest. It is understood engineering teams are still working to fix the issue, which could carry on through the weekend.
The IT problems have not only stopped accounts from updating balances but also prevented customers from getting onto their online banking systems.
Another source told Computer Weekly it is rumoured the problem is related to an upgrade. However, it is unusual for upgrades to banking IT systems to be carried out during weekday trading hours.
"If it is down until Monday, it is pretty bad because no NatWest customers can buy anything online," said the source.
A spokesman from RBS said: "We will continue working around the clock to fix the technical fault, but we are not going to put a timeframe on it, as we don't want to create expectations."
"We apologise to our customers and will get the issue fixed as soon as possible."
NatWest was named the most reliable banking website in 2010, when a study from Keynote Competitive Research said it was the only site monitored to have no errors on any page during peak hours.
