RBS and NatWest customers could face weeks of turmoil as a result of the banks’ computer chaos

NatWest was forced to further extend its opening hours today (Getty Images)

RBS and NatWest customers could face weeks of turmoil as a result of the banks’ computer chaos, the Financial Ombudsman Service warned today.

Meanwhile sophisticated internet fraudsters tried to cash in on the confusion by launching a phishing attack on NatWest customers in an attempt to steal crucial personal financial information.

And the pain continues for some 100,000 Ulster Bank customers who still have a frustrating wait until at least the beginning of next week to have their banking facilities restored.

The beleaguered Royal Bank of Scotland said today that just a few customers of NatWest and RBS will still experience delays after the update of account balances cleared for those banks.

But Ulster Bank customers will “continue to experience unacceptable delays to their accounts being updated” until “the start of next week”, RBS said in a statement.

The computer mistakes that have hit an estimated 12 million customers of the two banks, as well as having a knock-on effect on millions more, could leave problems for many weeks to come, the financial watchdog said.

The FOS’ David Cresswell urged those affected to keep detailed records of what has happened to their accounts as the knock-on effects would take some time to identify and rectify.

"It is really important to make noted of what is happening, of conversations you have with people, of difficulties you are facing, because this is a ripple effect from the original problems," he said.

That includes making lists of extra charges for inadvertently going overdrawn or paying a credit card bill late, and making sure that a your credit rating is not damaged.

NatWest said it has extended branch opening hours to 8am to 6pm in an effort to clear the backlog of problems.

"But the full focus of our efforts will now be on delivering the same result for our Ulster Bank customers,” an RBS spokesman said.

Customer unhappiness was mounting today with many expected to switch banks once their accounts have been restored.

Andrew Hagger of price comparison site Moneynet said there has been a surge of activity from people looking for information about rival bank accounts.

“In the last week, we saw the number of visitors to the current account section of the website increase by almost 10 per cent compared with the average for 2012 to date,” he said.

Meanwhile, crooks have been hoping to profit from the continuing confusion by sending out bogus emails purporting to be from RBS boss Stephen Hester.

They encourage customers to click on a link to a realistic-looking NatWest website and update their security information.

But anyone doing so will have their personal details stolen and used by fraudsters.

Bank of England Governor Sir Mervyn King today called for RBS bosses to be subject to a "detailed investigation" over the IT meltdown.

The Financial Services Authority refused to rule out enforcement action – such as a fine for the bank – but said: “We will expect RBS-NatWest to provide us with a complete account of the issues once this is fully resolved and to take any necessary steps to ensure that the risks of these problems occurring again are addressed."