Most Android handsets offer no easily accessible way of deleting user data, including access tokens, messages, images and other content, the study said.

Experts have been airing their concerns for some time now that the smartphones are extraordinarily difficult to clear of user data.

The study examined 21 second-hand devices running Android versions from five manufacturers that had been wiped using the operating system's built-in factory reset feature.

But the problems also exist with third-party data deletion applications, such as those offered by antivirus vendors, the researchers said.

The team was able to recover data including multimedia files and login credentials from wiped phones, and many of the handsets yielded the master token used to access Google account data, such as Gmail and Google Calendar.