Sir John Scarlett, who led MI6 for five years, voiced concerns over the growing use of tracking technology in phones and tablets that can monitor and log movements in minute detail.

Frankfurt - Star-based reviews are regular features in app stores, allowing buyers to signal whether they found an app useful or not.

But some unsavoury characters are starting to try to take advantage of such reviews. One trick is to promise buyers that all functions of an app will be unlocked once the user gives the programme a five-star review - the best possible - at the App Store or on Google Play.

Be sceptical of such offers. “That really can't be an above-board app,” says Martina Totz of the Consumer Centre of the German state of Rhineland-Palatinate. “

After all, this doesn't give you the chance to test the promised functions.” She says it's technically feasible to keep functions blocked until a five-star review is given. “You can certainly programme something that way,” she says.

But she notes that the user would have no idea what was being activated in the process. It's possible that they would simultaneously be unlocking a function that gathered data about the user.

Best to avoid such offers, she recommends. That said, there's no reason not to review apps that don't have strings tied to the review.

“This is very important feedback for serious designers,” she says. It's even more useful if buyers point out errors when making their reviews, so that they can be fixed in updates. It's also usually a good idea to read reviews before downloading an app.

Totz recommends focusing on the negative reviews. “That way you can see if an app gathers up too much data or if there are problems with updates.”

Sometimes, annoyed buyers even recommend alternate apps that work better or offer the same services for less money. - Sapa-dpa