RBS and NatWest borrowers are being advised to check their bank accounts following an admission by the RBS Group that some customers have had loan payments deducted twice.

RBS said it has charged some personal loan borrowers twice following the IT glitch that has caused chaos for hundreds of thousands of customers during the past two weeks. There are also as-yet-unconfirmed reports on Twitter that some of the group's 800,000 mortgage customers are also affected.

A spokeswoman for RBS said a "relatively small" number of customers have had two payments taken from their current accounts, although she admitted it wasn't yet clear how many were affected.

In a statement the bank said: "We apologise to any customers experiencing problems today. We said last week that we expected to see a few bumps in the road for customers as we get things fully back on track.

"Any customers experiencing problems should contact our call centre on 0161 931 9959 or 0800 656 9639, or visit their local branch and we will put things right."

One RBS customer contacting the RBS customer services department tweeted: "Personal loan payment to yourselves taken yesterday and today."

Another customer said: "Thank you RBS for changing my loan payment frequency to 'daily' without asking me!!!! How random!! I'll be bankrupt by Friday."

One customer was complaining he hadn't received his war pension because his building society's processing was done by RBS, while the Deposit Protection Service, a government-approved custodial scheme for tenants' deposits, said payments were still being delayed.

As the systems problems at NatWest and RBS complete their second full week and the Libor fixing scandal at Barclays claims the scalp of its chief executive Bob Diamond, customers have been expressing their disgust by switching banks. Nationwide building society said today it has seen an 85% increase week-on-week in the number of customers opening and transferring their main account online, while on Friday the Co-operative bank reported a 25% increase.

Nationwide head of current accounts John Crossley said: "Customers are clearly unhappy with recent events and are opting to vote with their feet."

Of 2,555 readers who voted in a poll on the Guardian website, 1,839 (72%) said they were fed up enough with the banks to switch accounts.