Customers trying to access both websites on Tuesday morning received an error message saying “RBS HTTP Error: 502” and a blank screen.

The IT failure leaves customers unable to access the banks’ websites or online banking login pages. However, online banking can still be accessed using the following direct links.

• Natwest: https://www.nwolb.com
• RBS: www.rbsdigital.com

According to fault monitoring website Downdetector.co.uk, Natwest.com started having issues at 9.25am and RBS at 10.02am on Tuesday.

Both banks posted the following message on their Twitter feeds: “Our website is currently unavailable. We’re working hard getting it back up and running for you. We’ll post an update when we have more information. In the meantime, please use our mobile app or contact us if you need urgent help.”

A RBS spokesperson said: “We are aware that customers are currently experiencing issues accessing our websites and online banking login pages. We apologise for the inconvenience caused. We are aware of the problem and are working hard to fix it. Customers can still access online banking via direct links, they can use their mobile app, ATMs, telephone banking, or visit their local branch.”

The outage comes less than a week after Natwest and RBS credit customers were among thousands of credit card customers affected by a hardware problem at US payments service provider TSYS.

Data from the Financial Conduct Authority shows that Natwest customers were hit by three incidents affecting internet banking in the past year. Barclays had the most incidents at 21, while Lloyds and Santander both had 15. Rules require banks to report incidents to the regulator if they meet certain criteria, such as affecting 25 per cent of the bank’s regular level of transactions.

The outage came the same day RBS re-launched its £150 current account switching offer.

Aashna Shroff, personal finance expert at money.co.uk, said: “It seems as though hardly a week goes by without a high street bank failing its customers by experiencing an IT outage for some reason or other. Today, it’s the turn of Natwest and RBS. It’s just not acceptable that consumers can’t rely on their banks to give them access to their money when they need it.

“If you are out of pocket as a result of Natwest and RBS being offline – for example, because you couldn’t make a critical payment – the banks should ensure you are not penalised with additional charges for late payments or interest, either cancelling the charge or refunding the amount if another bank charged you.

“Similarly, if your credit rating has been affected because you’ve missed a payment, the banks should help you resolve this. Remember to check your credit rating to see if there have been any issues and contact your bank immediately if there are. If there’s any quibble about any of this, you can always take your case to the Financial Ombudsman – whose website is up and running.”