The Halifax online banking system, IF, was due to go live last Friday, but has now been put back until August.

While the system coped in an automated test environment, it experienced problems when tested live by 500 call centre staff.

The Halifax estimates it will need to be able to deal with a possible peak of 15,000 customers per day.

IF's embarrassment comes just weeks after the Abbey National's online bank, Cahoot, crashed on launch due to high customer demand.

An Abbey National representative said it was surprising that the IF launch was pulled so late, and cast doubt on IF's explanation. "The problems IF are experiencing may not just be due to high volume, and possibly indicate something more fundamental," said Mac Millington, the bank's managing director of business-to-consumer banking.

A spokesman for IF denied the problem was more serious than it had told its customers. "When the system was moved into the live environment, further tests revealed a performance issue that, while not fundamental, may have adversely impacted customer service under high volume conditions. Work is now being carried out to resolve this issue."

Analysts blamed over-ambition, but said the Halifax had made the right decision to postpone the launch until it was fixed. "It is difficult to predict the load-balancing in the live environment, as the geography is very hard to mirror," said Antony Smyth, partner of systems assurance and advisory services at PricewaterhouseCoopers.

However, he added that a large European bank he was working with was doing three months testing in a live environment with 1,000 customers before even advertising the service.