The Royal Bank of Scotland (RBS) has repaid the 600,000 customers who suffered on Wednesday when payments into their accounts failed to show up, the banking group announced this morning.

The bank blamed the glitch on a technical error, as a package of incoming payments was not processed correctly, leaving RBS and NatWest customers out of pocket.

“We can confirm that the issues customers were experiencing in relation to delayed credits and debits have now been resolved and accounts have been updated. We are extremely sorry for the inconvenience and distress that this has caused our customers,” the bank said in a statement.

RBS chief Ross McEwan

RBS chief Ross McEwan has promised to regain customers’ trust.

“If any customers are still experiencing issues please contact our call centres or come into a branch where our staff are ready to help. We will continue our work to make sure that no customer will be left out of pocket as a result of this issue.”

The group had initially said customers would receive the payments by Saturday, but it has managed to resolve the situation one day ahead of schedule.

RBS has not yet worked out who is to blame for the problem, and whether the cause was an internal error or a problem with the incoming data. A spokesman said the bank wants to make sure customers are looked after first, before moving on to assigning responsibility for the glitch in the coming weeks.

To that end, it has drafted hundreds of extra staff into branches and call centres in order to deal more quickly with customer queries and complaints.

Yesterday, the bank’s chief administration officer, Simon McNamara, told investors that £3.5bn will be pumped into IT systems over the next two years and that this investment would help prevent similar errors from happening again. Of that, £150m per year has been earmarked for “building resilience into the system”.

Royal Bank of Scotland RBS

RBS has suffered a series of high-profile malfunctions in recent years. Last year, the bank was fined £56m after a software issue in 2012 left millions of RBS, NatWest and Ulster Bank customers unable to access their bank accounts.

It was then hit by another glitch the day after paying the fine.

Even its newer systems have been beset by problems. In 2013 its mobile app failed 34 times, though Mr McNamara said further investment has reduced its glitch rate to just one so far this year.

On Tuesday, Ross McEwan, RBS’s chief executive, told investors and analysts he wanted his company to become the number one bank in the UK for “customer service, trust and advocacy” by 2020.

Alongside the programme of improvement work to patch up the bank’s creaking network of legacy IT systems, RBS is also building new platforms, including an app to allow customers to access services through the Apple Watch gadget.