RBS has been hit by another embarrassing IT failure, which left users of its popular mobile app unable to access their accounts online.

Two million customers at the taxpayer-backed bank, which includes NatWest, have the app on smartphones and tablet computers, and many took to Twitter to express frustration after they were unable to use it to check their money.

RBS apologised over the glitch as it scrambled to restore the service. A spokesman later said the app was working again. It appeared to have been out of action for two-and-a-half hours.

The failure follows an IT fiasco last summer in which payments went awry, wages appeared to go missing and home purchases and holidays were interrupted. It has already cost the bank £175 million in costs and compensation.

The setback yesterday was the latest bump in the road for RBS, which is 81% publicly owned, as hopes rise it can soon be returned to the private sector after being rescued at the height of the financial crisis.

The new glitch is particularly embarrassing as the bank is looking to shift its focus towards online customers.

Frustrated customers trying to use the app were told there was no internet connection, although other apps were working.

One said he had been logged out in the middle of a transaction. Another tweet said: "Pointless having the £rbs app because it hardly ever works."

RBS Help responded on Twitter, saying: "We are aware of some issues on our Mobile Banking service and are working hard to fix them. Sorry and thanks for your patience."