The state-backed British lender said the cause of the problem had been identified and it was working to resolve missed wages, benefits, bills and other payments for customers at its RBS, NatWest, Coutts and Ulster Bank brands in Britain and Ireland.

The bank, which has been hit by a number of technical problems in recent years, advised concerned customers to contact their branches or the bank's call centres and said no customers would be left out of pocket.

"We have fixed the underlying issue, we apologise for the inconvenience caused and we are working flat out to get these payments updated for our customers no later than Saturday," it said.

Conservative lawmaker Andrew Tyrie, chairman of parliament's Treasury Select Committee, said that response looked "unacceptable".

"Restoration of payments should be a top priority. It is crucial for those in the greatest financial need and also those who find it difficult to go to a branch," Tyrie said.

Tyrie said he would write to Ross McEwan, RBS's chief executive, about the matter, as well as the head of Britain's Prudential Regulation Authority and Financial Conduct Authority.

"These failures should concern both the conduct and prudential regulators," he said.

The Bank of England said it was in contact with RBS and would be working closely with the bank as it resolves the problem.

RBS was fined 56 million pounds ($88 million) last year by Britain's financial regulators for a system breakdown in 2012 that left 6.5 million customers unable to make or receive payments for days.

The bank said it would invest an extra 750 million pounds in the security and resilience of its IT systems by the end of 2015 to prevent a recurrence of its past problems.

The latest failure prompted an angry response from customers on twitter.

"Can't get gas or electric as have no money, three kids. Not good enough. Nothing but problems with this bank, going to change," one customer complained.

Another said: "Once again no money due to major mess up. Tax credits not paid into my account. No food, no electric, single mum with three kids".

Some industry sources say RBS's systems are outdated and made up of a complex patchwork of systems after dozens of acquisitions.

Chief Administration Officer Simon McNamara was due to give an update on the bank's IT strategy on Thursday.

(Additional reporting by David Milliken; Editing by Simon Jessop and Susan Fenton)

By Matt Scuffham and Steve Slater