Google has confirmed the Pixel 4 smartphone's Face Unlock system can allow access to a person's device even if they have their eyes closed.
One security expert said it was a significant problem that could allow unauthorised access to the device.
By comparison, Apple's Face ID system checks the user is "alert" and looking at the phone before unlocking.
Google said in a statement: "Pixel 4 Face Unlock meets the security requirements as a strong biometric."
Speaking before the launch, Pixel product manager Sherry Lin said: "There are actually only two face [authorisation] solutions that meet the bar for being super-secure. So, you know, for payments, that level - it's ours and Apple's."
On Tuesday, BBC News tested the Face Unlock feature on the new Pixel 4.
Using the default settings, the phone still unlocked if the user pretended to be asleep.
The test was repeated on several people, with the same result.
Images of the Pixel 4 leaked before launch showed a setting labelled: "Require eyes to be open," in the facial-recognition menu.
However, this setting was not present on the devices loaned to BBC News.
And Google told BBC News it would not feature on the Pixel 4 when it went on sale, on 24 October.
"If someone can unlock your phone while you're asleep, it's a big security problem," said cyber-security expert Graham Cluley.
"Someone unauthorised - a child or partner? - could unlock the phone without your permission by putting it in front of your face while you're asleep," he told BBC News.
"I wouldn't trust it to secure the private conversations and data on my phone."
Google's Pixel 4 support website tells customers: "Your phone can also be unlocked by someone else if it's held up to your face, even if your eyes are closed."
It says concerned customers can switch on "lockdown" mode - which deactivates facial recognition - when they want enhanced security.
Google told BBC News Face Unlock could not be fooled by photos or masks, however.
"We will continue to improve Face Unlock over time," it said in a statement.