Royal Bank of Scotland has suffered another IT fiasco after admitting it could take until the weekend for customers to receive 600,000 payments that failed to enter accounts overnight.

RBS said the affected payments to and from customers’ accounts would be made by Saturday at the latest, as account holders bombarded the bank with complaints overmissing transactions including wages, tax credits and disability allowances.

The embarrassment comes seven months after RBS was fined £56m for an IT meltdown in 2012 that left 6.5 million customers locked out their accounts for days. Customers missing their payments on Wednesday included account holders at NatWest and Ulster Bank, which are both owned by RBS. Clients of RBS’s elite Coutts bank, where the Queen is reputed to have an account, were also affected.

The problem became evident when the RBS Help account tweeted: “Some customer payments are missing this morning – we are investigating this issue as a matter of urgency.” The bank told affected customers who needed funds urgently to get in touch.

Andrew Tyrie, chairman of the Treasury select committee, said delaying the delivery of some payments until Saturday was “unacceptable.” Warning that the bank was not acting quickly enough, he said: “RBS is suggesting that the delayed payments will be processed ‘no later than Saturday’. This looks 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.”

One customer affected by Wednesday’s glitch said she had not received hundreds of pounds in benefits. Lynne Kenny, 52, an RBS customer for 30 years, said the bank’s failure meant she did not receive her widow’s pension and working tax credit totalling about £500.

“I called my branch in Derby and they said I would have to come in to sort it out but my father passed away on Sunday and I’m in the middle of funeral arrangements. They said I would have to ring the complaints number. When I did, they said they didn’t expect to be able to put that money into my account today but I said I need money for food so they paid in £50.”

RBS Help (@RBS_Help) Some Customer payments are missing this morning – we are investigating this issue as a matter of urgency. 1/2

Other customers told the bank via Twitter they were worried about important payments not being made.

Joanna Lawson (@jolawson73) @RBS_Help My monthly Asda delivery has been cancelled due to no funds in account, I was relying on it.

Dr Sharon Connor (@SharonConnor3) @RBS_Help DWP payment not due today has not been received.

Jack Admans, 20, was stuck in Ashford Station after his account at thinkmoney, which uses RBS systems, failed to deposit his monthly wage. “Caught a train from Reading to Kent today at 8am for a job interview at 12. I was relying on my wages to be able to pay for the train journey. I’m stuck at the train station in Kent with no money and I am not allowed to leave the station without paying.

“I have missed my job interview. This was a once in a lifetime overseas job that I was interviewing for. It was a selection day and now I have missed out. I’m gutted.” Thinkmoney said it expected delayed payments to be come through by Friday at the latest.

A spokeswoman for RBS said the payments were not missing but were delayed. She said the computer problem that caused the delays had been fixed and that customers would not lose money as a result of the glitch.

The bank said: “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.”

“To any customers concerned about the implications of this issue we advise them to come into a branch or get in touch with our call centres.

Wednesday’s episode is the latest system failure to hit the taxpayer-owned bank after an incident that left millions of customers shut out of their accounts in June 2012. The bank was fined by the City regulator last year over that incident and has since pledged to spend £750m to prevent a similar incident occurring.

Tyrie said he would write to RBS’s chief executive, Ross McEwan, and banking regulators calling on them to make sure there was an end to such failures. RBS’s previous chief executive, Stephen Hester was forced to forfeit his annual bonus over the IT failure in 2012.

The Financial Conduct Authority’s RBS team spent Wednesday checking on how RBS was dealing with the problem.

The regulator said: “The FCA is in contact with RBS. We will be working closely with RBS as it resolves the problem and to ensure there are steps in place to help consumers who are affected.”

The regulator told banks to make sure their computers are able to support millions of their customers’ accounts after RBS’s earlier failure highlighted the antiquated state of many of their systems.

Lloyds and Nationwide are among lenders to have had problems since, with the latter’s customers hit by online payment delays this week.

Have you been affected by RBS’s IT glitch? Share your experiences using the form below. We’ll use a selection in our reporting.