Ulster Bank admitted this evening that it doubled charged some of its customers for transaction in recent days as it worked to resolve a major technical failure which has left as many as 600,000 customers without access to funds for close to three weeks.

The bank was unable to say how many accounts had been affected by its latest error but it apologised to affected customers and said they would not be left out of pocket as a result of the double charging.

"As we work to restore normal service, some customers may have noticed duplicate transactions on their accounts," a spokeswoman told The Irish Times. "We apologise for this and can assure customers that this is being rectified and that all duplicate postings will be reversed.

In a seperate announcement the bank said the majority of the 600,000 customer accounts impacted by technological problems have now been updated as of July 2nd.

In a brief statement issued this evening Ulster Bank said significant progress had been made in updating customer accounts.

"The majority of our systems are showing balances as of Monday July 2nd," a spokeswoman for the bank said. She added that while all the bank’s ATMs were working, some were showing incorrect customer balances.

Approximately 50 Ulster Bank branches are open until 7pm. Some 36 of the bank's branches will be open tomorrow from 9.30am until 3pm with some also opening between 10am and 1pm on Sunday.

The bank said it has also doubled the number of staff available at its call centres in order to assist customers.

The lenders is facing an unquantified level of compensation claims from customers who have been inconvenienced and suffered losses as a result of an unprecedented technical crisis that has affected tens of thousands of customers for a third week.

More than 600,000 customers have been without access to funds for more than two weeks, and problems are expected to continue into next week.

Speaking earlier today financial ombudsman Bill Prasifka said it was too early to judge if Ulster Bank's response was adequate.

Mr Prasifka said complaints would be looked at on a case by case basis, but as the problems had not yet been solved, it was difficult to judge what the economic and potential consequential loss of the computer glitch amounted to.

"The problem is not over, things have to work themselves out. The goal of our office has always been not simply to make more and more adverse rulings against the financial institutions, but to see them deal with customers, to see them rectify problems when they arise," he said. "This will be a test."

His office had only received about a handful of complaints about the situation, although there had been plenty of queries. Mr Prasifka said the complaints would be given to the bank to deal with first.

He said there would be many difficult cases that would arise from this, citing stories of people unable to close sales of houses, and the failure of direct debits to be paid into united linked funds, which would give rise to complicated issues of consequential loss that would need to be worked through on a case by case basis.

Ulster Bank has promised that no one would be out of pocket as a result of the problem. The bank confirmed yesterday that compensation would be offered to customers affected by the technical problems.

Mr Prasifka said this would be taken into consideration when awarding compensation to customers. "Our statutory scheme is one based on compensation. We're not there to impose penalties on institutions," he said.

Compensation could cover out of pocket loss, and also stress and inconvenience arising out of maladministration. However, Mr Prasifka told RTÉ radio it would not cover legal fees should consumers engage a solicitor to make their complaint to the Ombudsman.

The bank's chief executive Jim Brown yesterday bowed to political pressure and agreed to waive his annual bonus for 2012. The New Zealand banker was appointed chief executive of Ulster Bank last year by the bank’s UK parent company, Royal Bank of Scotland.

The Irish Bank Officials Association said today it welcomed the decision by Mr Brown to waive a bonus this year but said the handling of the crisis “raised serious concerns”.