Users found their wake-up alert coming one hour late, one hour early or not at all.
The problem, related to the clocks going forward for British Summer Time, does not appear to have affected everyone.
Apple has yet to comment on what caused it, but similar problems have previously hit iPhones in the US.
Many of those whose alarms went wrong turned to social networking sites to vent their fury. One Twitter user wrote: "iPhone alarm failed twice. 1) went off at 5.45 instead of 6.45. 2) Didn't go off at all when I reset it. Time to update software."
Another unimpressed owner wrote: "Thanks iPhone. I didn't really want that alarm to go off anyway."
According to user reports, the glitch has affected non-recurring alarms set within the iPhone's calendar application, rather than its dedicated alarm clock.
The problem first came to light in the United States last November during the switch from Daylight Savings Time.
Despite promises from Apple to correct it, a similar issue hit iPhones on 1 January 2011.