RBS shareholders at yesterday's annual meeting may not have been overly worried about last week's IT systems malfunction, but for 600,000 customers it was yet another let down.

The complete breakdown of its platform three years ago left 6.5 million customers suffering disruption lasting, in some cases, for several weeks.

Online banking disappeared, cash machines could not provide balances, mortgage payments were delayed and customers received inaccurate bank statements.

The then chief executive, Stephen Hester, had to explain himself before the Treasury Committee of MPS and even waived his 2012 bonus to show contrition.

With the bank under scrutiny for how it would respond, two smaller glitches during 2013 served to rattle customer confidence. In March, cash machines failed to respond properly, and in December, on the busiest shopping day of the year, customers were locked out of their accounts for three hours.

On that occasion, newly-arrived chief executive Ross McEwan blamed "decades of under-investment" in the systems and said the bank was now "investing heavily in building IT systems our customers can rely on".

To make sure the situation would not happen again, RBS said last year it was investing £750 million in the security and resilience of its IT systems.

Last November the regulators slapped fines of £56m on RBS for the 2012 debacle, on top of the £175m paid out in customer compensation.

Last week's episode saw payments into accounts delayed for up to two days, prompting Treasury Committee chairman Andrew Tyrie to describe it as a serious issue.

At yesterday's meeting Mr McEwan appeared to assure shareholders that RBS had already spent £750m, and the job was as good as done. Chairman Sir Philip Hampton, meanwhile, attempted to shrug off the idea that RBS was in any way under-invested by saying that the Financial Conduct Authority had given it a clean bill of health, even in 2012. He added for good measure that only five per cent of the bank's daily transactions had been affected.

Meanwhile Mr McEwan continues to avow that the bank is on track to achieve its core ambition of being the number-one bank for customers. He said work was going on behind the scenes to simplify and improve systems and processes, to make them reliable, easy to use and much more efficient.

He is certainly leading a shift in the bank's culture, putting the emphasis on fair and transparent savings accounts and credit cards, shorn of gimmicks and temporary bonuses, setting a target of 30 per cent female representation in the bank's top 600 roles and launching entrepreneurial hubs led by the one at Gogarburn in a wing once occupied by Fred Goodwin's suites.

There were even olive branches offered to the bank yesterday by some of its most embittered critics, those small business people who believe RBS set out to ruin them. Their stinging critiques of Sir Philip's regime were tempered by compliments to the open listening style of Mr McEwan.

But with the Government about to embark on its first sale of RBS shares, and Lloyds leading the way in the privatisation race, RBS will have to up its IT game if it is to convince investors that it is fully ready for a quick return to the private sector.