"Our findings call into question the recent claim by Apple that the iPhone 4's signal-strength issues were largely an optical illusion caused by faulty software that 'mistakenly displays 2 more bars than it should for a given signal strength.' Consumer Reports said.

The comments by the product-reviews publication add to a pile of complaints about the iPhone 4's ability to handle voice calls. Apple has responded with a promise for a software update that will change how the phone decides how many signal bars to show, but Consumer Reports' tests cast doubt on whether that will solve the problem.

Consumer Reports tested three iPhone 4s and found that touching a spot on the left side of the phone can cause reception to "significantly degrade enough to cause you to lose your connection altogether if you're in an area with a weak signal."

"Due to this problem, we can't recommend the iPhone 4," the magazine said in a statement. The conclusion was an about-face for Consumer Reports, which said in a July 2 blog post that the new iPhone's issues weren't yet a reason to forgo buying the device.

The newest version of the iPhone is the first that the magazine has failed to recommend. Apple didn't reply to a request for comment.

In an open letter to customers earlier this month, Apple said the phone lost signal bars because it was showing a too-strong signal to begin with. While the iPhone might show too many bars as Apple says, "it's a hardware issue, because when you touch the hardware, it's affected," says Consumer Reports editor Mike Gikas. "There is definitely a problem."

Apple has recommended that customers put rubber protective sleeves, or "bumpers," around the phone, a step that Consumer Reports also endorsed. The magazine also suggested covering the gap in the antenna with a piece of duct tape. "It may not be pretty, but it works," the magazine said.

The publication also took some of the heat away from AT&T Inc.'s network, which has been criticized for not being able to adequately handle the load of data-guzzling iPhones. "The tests also indicate that AT&T's network might not be the primary suspect in the iPhone 4's much-reported signal woes," Consumer Reports said.

Write to Niraj Sheth at niraj.sheth@wsj.com