Royal Bank of Scotland (RBS) said on Tuesday it will compensate customers who lost money owing to a "systems issue" which prevented them from using their payment cards on one of the busiest shopping days of the year. The troubled lender said it had fixed the glitch which had left customers unable to pay for petrol after filling up their cars -- in principle a criminal offence -- and others forced to abandon trolleys full of shopping at the checkout. "We've really caused problems. It is completely unacceptable that customers couldn't access their own money," Susan Allen, RBS director of customer relations, told BBC radio. The state-rescued bank, which owns Natwest and Ulster Bank, added in a statement: "The systems issues that affected our customers last night have now been resolved and all of our services are now back working normally. "If anyone has been left out of pocket as a result of these problems, we will put this right." Customers used Twitter to vent their anger about the latest problem, which occurred on so-called 'Cyber Monday', tipped to be the busiest online shopping day of the year as shoppers took to the Internet to buy Christmas presents. "Just had to leave an IOU at the petrol station," wrote one customer, while another tweeted: "NatWest_Help just topped up a full tank of petrol only to find out there are technical difficulties with all NatWest cards." It was the latest in a string of technological meltdowns at RBS, most recently when an IT failure in may left customers unable to access their accounts online using mobile apps. In June 2012, a software upgrade left hundreds of thousands of people unable to make or receive payments for several days, and cost the group £175 million (210 million euros, $290 million) in compensation. The bank has struggled with a series of blows to its reputation since it was rescued by the British government following the 2008 financial crisis. Most recently, a report by the government, which owns 81 percent of the bank, accused RBS of forcing small businesses to default so that it could sieze their assets.