Customers logging onto Natwest mobile or online banking this morning were greeted with a message saying: “Some customer accounts are showing debit card purchases twice, and in certain cases their available balance will be lower than it should, we’re really sorry for any inconvenience.”

Natwest says affected customers won’t be left out of pocket by the issue. The bank said payments had only been made to the retailer once, so there was no need to contact relevant retailers.

Natwest said customers needing help should either drop the bank a private message on social media, message the bank through the banking app, or chat to it online. However, it warned that customer support might be busier than normal.

But Natwest’s apologies weren’t enough to pacify some customers who took to social media to complain.

One customer Tweeted: “@NatWest_Help I paid £65.50 online few days ago and payment cleared. Now same transaction is showing as pending this morning. Same company and same amount? Is this due to your issues and if so, when will you sort this?”

Another Tweeted: “@NatWest_Help My balance is also appearing much lower than expected, but I cannot immediately see any double transactions. Could you please advise?”

Another Twitter user said: “@NatWest_Help two days now duplicate transactions showing in my account and putting me under stress purchasing anything as I may be put in overdraft. What are you doing to fix this and when will I get money back this is ridiculous!!”

Replies from @Natwest_Help suggested the affected payments were initially paid on 7 May.

A Natwest spokesperson said: “A small number of customer accounts showed Visa debit card purchases twice. The problem is now resolved and we will ensure that no customer is left out of pocket.”