A nonprofit organisation, Consumer Reports, which publishes guides on everything from cars to TVs, reported that iPhone 4 has the signal-loss problems.
The organisation also tested tested other phones including the iPhone 3GS and Palm Pre but these devices do not have this problem.
The report was the latest blow to the iPhone 4 which has been hassled by complaints of poor reception. Many of the complaints involve a wraparound antenna whose signal strength is said to be affected if touched in a certain way.
"When your finger or hand touches a spot on the phone's lower left side, an easy thing especially for lefties, the signal can significantly degrade enough to cause you to lose your connection altogether if you're in an area with a weak signal," said Mike Gikas, team member of Consumer Reports.
"Signal problem is the reason the iPhone 4 would not be classified as a recommended device in its smartphone ratings," he added.
However Apple Inc did not respond to the latest Consumer Reports.