The failure last Monday of the new swipe-card system lasted from 12.30pm to 3.30pm on one of the busiest days of the week for picking up pensions.
A Post Office spokesman said it was too early to release information about the cause of the problem. "Until we have a full breakdown it is difficult to give out that information. We are concentrating on making sure that payments work, and making sure that Bacs payments go through. People are testing the system trying to make sure that it is as robust as possible," he said.
A spokeswoman for JP Morgan confirmed that the delays in pension payments were caused by a system it was managing as a sub-contractor for EDS. She added that it was still investigating the cause of the technical failure in partnership with the Post Office and other IT suppliers.