Royal Bank of Scotland customers faced frustration today after the bank suffered another IT failure.
Customers attempting to use NatWest, RBS and Ulster Bank mobile apps found they were locked out of the service this morning.
Around two million people use the NatWest service alone and the problem also affected customers of RBS and Ulster Bank, which are part of the same group.
The technical problems are thought to have started at around 7am this morning and follow other glitches that beset the group, starting with a huge IT meltdown last summer.
A spokeswoman for RBS could not say how many customers have been affected, although both personal and business users were hit by the issues.
The news of internet connection problems comes amid reports that the internet worldwide has been slowed down by what experts are describing as the 'biggest cyber attack in history'.
The attack is believed to have begun last week, when Spamhaus, an anti-spam organisation, was hit by a wave of digital traffic that knocked its website offline.
Spamhaus, which blocks spam from email accounts, blacklisted a Dutch hosting company leading to a furious online row.
The quarrel was followed by a number so-called DDoS attacks, cyber assaults that overwhelm servers with requests.
Yesterday experts warned that the attack had slowed down the speed of the internet worldwide and that it could go on to have an impact on email and banking services.
Despite the timing, the attack is not thought to have caused today's mobile banking service problems.
These latest IT banking issues come just three weeks after a hardware fault prevented RBS Group customers from using cash machines and also affected online and telephone banking services.
The group took a £175 million hit as a result of last summer's IT chaos which left NatWest, Royal Bank of Scotland and Ulster Bank customers locked out of their accounts.
The bulk of the redress costs related to Ulster Bank, where the problems took weeks longer to clear up. A statement from RBS Group earlier said: "We are aware of a technical problem this morning which is preventing customers from logging in to our mobile banking applications.
"We are working to fix the problem and apologise to customers for the inconvenience caused. No other systems are affected."
The collapse also comes just weeks after the bank unveiled a new mobile banking feature for RBS/NatWest customers called Pay Your Contacts, which allows them to send payments of up to £100 to anyone with a Visa card, by entering their mobile number.
Last October, NatWest had to suspend another feature on its mobile phone app called GetCash, after the service was subject to a spate of "phishing" attacks by fraudsters.
The GetCash service, which allows customers to withdraw cash without using a debit card, was later re-instated after security was beefed up.
A statement from RBS later confirmed all applications were back up and running: "All our mobile banking applications are now running normally. We apologise again for the inconvenience caused to our customers", the bank said.