Scores of British iPhone 4 users said they were late for work on Monday after a software bug meant the alarm on the Apple device failed to adjust when the country's clocks changed.
Hundreds of angry comments were posted on microblogging website Twitter about the apparent glitch, which happened even though the rest of the phone's features updated the time automatically.
"Well done Apple -- you've made me decide I need to use a proper alarm clock rather than relying on my iPhone," one user tweeted.
"Stupid iPhone alarm clock went off an hour late. What a great start to the week," said another.
Britain put its clocks back by one hour on Sunday from British Summer Time to Greenwich Mean Time.
There was no immediate response from Apple.
But the tech giant said last month that a software patch was being developed when Australian iPhone users had similar problems -- though their alarms went off an hour early as the country was switching to daylight saving time.
The latest fault can apparently be avoided by using one-off alarms instead of pre-set daily wake-up calls.
The iPhone 4 suffered a turbulent launch after problems with its antenna, while the launch of the white version was recently postponed again until next year.
But Apple has described the iPhone 4 as its most successful product launch ever, with more than three million sold in the first three weeks after its debut.
Apple said last week that it sold 14.1 million iPhones during the latest quarter, up 91 percent from a year ago.
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