State-rescued Royal Bank of Scotland was battling on Monday to repair an already battered reputation as it struggled to fix a week-old computer glitch that has affected millions of customers. Software problems that left customers at the bank and at RBS-owned lenders NatWest and Ulster Bank unable to pay bills, access accounts and receive wages have been fixed -- but a huge backlog of unprocessed transactions remain. \"We are checking our systems... and speaking to customers,\" an RBS spokesman said Monday, a day after the bank told AFP that a \"large majority\" of the three lenders\' combined 15 million personal banking customers had been affected by the IT meltdown. NatWest in a separate statement apologised to customers for causing \"an unacceptable level of inconvenience,\" adding that more than 1,000 of its branches would remain open on Monday until a later-than-usual 1800 GMT. Ulster Bank, which serves clients in Northern Ireland, said on its website that about 80 branches would have extended opening hours on Monday. \"We recognise this is an unacceptable inconvenience and we want to reassure our customers that if they experience bank charges as a result of this issue they will be refunded. Once again we would like to unreservedly apologise,\" Ulster Bank added. RBS chief executive Stephen Hester meanwhile issued a public apology on Saturday and admitted that customers had been let down. Hester said double the usual number of staff were manning call centres to tackle the problem, which reportedly arose after a failed attempt to install an update on RBS\'s payment processing software. Despite the banks\' moves, customers continued to vent their anger on social networking websites. \"Goodbye NatWest, from a customer of 25 years\' standing,\" read one comment posted Monday on Twitter. Parent group RBS is 82-percent owned by the British government after a massive bailout in the wake of the global financial crisis. But it is still struggling to repair itself and last month posted a first quarter net loss of £1.52 billion (1.87 billion euros, $2.46 billion), almost three times the amount posted a year earlier. Amid public anger over a raft of poor results, Hester waived his latest annual bonus of shares worth £963,000, which had been due on top of his £1.2 million salary for his work in 2011. RBS last week meanwhile said it planned to axe 618 jobs at its department offering financial advice to customers, on top of 35,000 posts axed by the group since the 2008 financial crisis. \"Unfortunately for RBS the damage is irreparable for the customers that they will lose as a result (of the glitch) and the problems will only feed anti-baking rhetoric further,\" said Simon Denham, head of Capital Spreads trading group. Investec bank analyst Ian Gordon said the fiasco could cost RBS dearly. \"I would estimate that the cost could run into the tens of millions of pounds,\" he said. \"If you ask about the reputation damage, that is harder,\" to estimate. RBS shares were down 1.19 percent at 240 pence on London\'s benchmark FTSE 100 index which showed a drop of 0.80 percent at 5,469.44 points in afternoon deals.