Google has reinstated its Street View cars following the admission they had mistakenly collected Wi-Fi data.

The cars tour cities across the world taking photos for use on the search engine's Street View service - an add-on to Google Maps, which offers real photographs of locations.

However, in May, it was revealed that the cars had accidentally also collected data including SID information (the network's name), MAC addresses (the number given to Wi-Fi devices such as a router) and payload data such as emails or web page content being viewed from unsecured Wi-Fi networks.

The error came to light after the German data protection authority audited the Wi-Fi data collected.

As soon as Google became aware of the problem it grounded the cars and then separated the data and disconnected it from the network.

However, the search engine revealed the cars were now back on the road.

"We have decided to start Street View driving in Ireland, Norway, South Africa and Sweden again starting next week," said Brian McClendon, vice president of engineering for Google Geo, in a blog.

"Our cars will no longer collect any Wi-Fi information at all, but will continue to collect photos and 3D imagery as they did before.

"We recognise that serious mistakes were made in the collection of Wi-Fi payload data, and we have worked to quickly rectify them. However we also believe that Street View is a great product for users, whether people want to find a hotel, check out a potential new home or find a restaurant."

Following a complaint from Privacy International, the Metropolitan Police are investigating the search engine's error.