The Royal Bank of Scotland (RBS) has issued an apology to the 17.5 million customers that are thought to have been affected by the banking group's recent computer woes.
"I am very sorry for the difficulties people are experiencing," Stephen Hester, group chief executive of the Royal Bank of Scotland, said in a statement.
This is an unacceptable inconvenience for our customers, for which we apologise.
"Our customers rely on us day in and day out to get things right, and on this occasion we have let them down. This should not have happened."
Initially, there were claims the problem occurred when the bank's computer software was updated.
A spokesperson for the banking group said the problem occurred when existing software failed to carry out overnight transfers of money between accounts.
This caused some customers to miss rent, mortgage and salary payments, it has been claimed.
An update on the RBS website stated: "The main problem customers are having is that where people have had money go into their accounts overnight, there may be a delay in it showing up on their balance.
"This is an unacceptable inconvenience for our customers, for which we apologise."
RBS, which owns NatWest and Ulster Bank, assured customers the computer problem had been fixed over the weekend.
In the meantime, RBS has promised to automatically waive overdraft fees on existing accounts.
The banking group will also work with credit agencies to make sure that no one's credit score is adversely affected by the outages.
RBS will also reimburse customers for the cost of calling their helpline number.
"We have doubled the usual number of staff in our call centres, and for the first time ever we [opened] 1,200 branches across the country on a Sunday from 9am to 12pm," Hester added.
Although the computer glitch was fixed last Friday, there is still a backlog of payments for the banks to get through.
Susan Allen, director of customer services for RBS Group, confirmed yesterday that "we are making progress to clear the backlog of payments and I'm cautiously optimistic that RBS and NatWest customer account balances will be largely back to normal from tomorrow.
"For all other issues, customers have our commitment that they will not be out of pocket from this issue. We will publish further details on how we will ensure this later in the week," she added.
Despite these improvements, Ulster Bank's 1.9 million customers may have to wait until the end of this week for their accounts to return to normal.
Ulster Bank reports that about 80 Ulster branches will stay open until today at 6pm to deal with the backlog.
"This is taking time," Hester said. "But I want to reassure people that we are working around the clock to resolve these problems as quickly as we are able."
