Taxpayer-owned Royal Bank of Scotland is facing growing unrest as it admitted tonight that it will be “Monday at the earliest” before it will sort out the computer problems that have brought financial chaos to millions.

The computer glitch which hit NatWest and RBS customers on Wednesday has meant salaries and payments have not being credited to accounts, leaving customers unable to get cash and having debit card transactions turned down.

But the problem has now spread beyond the estimated 12m customers of the RBS group, including those with accounts at Ulster Bank.

Workers at firms which use RBS or NatWest to make staff payments say their salary hasn’t been paid in, while a house purchase was reported to have collapsed today because the buyer’s solicitor banked with NatWest.

One disgruntled customer of a building society turned to RBS’s own website to vent his feelings. FredA wrote: “I’m with Norwich & Peterborough and my wages come from RBS. I am about to get £139.50p in bank charges for returned direct debits. Will RBS cover my costs?”

The bank had been quick to reassure its own customers than none would be left out of pocket as a result of the continued cock-ups.

In a statement, the bank said: “We will ensure that no customers will be permanently out of pocket.”

However it remained unclear last night whether RBS would reimburse customers of rival banks who have been adversely affected by the problems.

After experiencing growing complaints and queues yesterday, the bank kept around 1,000 branches open to 7pm to help customers.

Many had had cashcards rejected at ATMs or in stores because of a supposed lack of funds in their account.

At branches, however, they were able to withdraw money from their own accounts as staff could manually check records and see that salaries or other payments had been deposited.

The bank’s branches will remain open until 6pm tomorrow and, in an unprecedented move, will also be open on Sunday - from 9am to 12pm.

The bank said: “We are continuing to experience technical issues with our systems, which is impacting a large number of our customers. As a result, money credited to accounts overnight may not be appearing on balances. This problem is strictly of a technical nature and we are working hard to fix it as soon as possible.

“We recognise this is an unacceptable inconvenience for our customers, for which we apologise.”

It has become clear that the problems have not only affected the bank’s current account holders, but those with savings and offset mortgages, too.

Customers affected have been warned to keep a record of what happened and how they were left out of pocket.

The Financial Ombudsman Service said banks are obliged to return their clients to the position they would have been in had the problem not existed.