Google has been threatened with criminal proceedings by the information commissioner's office (ICO) over data secretly collected by its Street View cars in the UK.

The privacy watchdog said it would prosecute the US firm under the contempt of court act if it failed to delete private information it gathered from public Wi-Fi networks.

The ICO has served Google with an enforcement notice ordering it to delete the data within 35 days or face criminal proceedings.

Stephen Eckersley, the ICO's head of enforcement, said: "Today's enforcement notice strengthens the action already taken by our office, placing a legal requirement on Google to delete the remaining payload data identified last year within the next 35 days and immediately inform the ICO if any further disks are found. Failure to abide by the notice will be considered as contempt of court, which is a criminal offence."

It is not known what private details are included in the so-called payload data held by Google about British citizens. But privacy watchdogs in the US found that it contained traces of passwords, emails and text messages sent over unsecured Wi-Fi networks.

Google has faced criticism from regulators around the world for collecting private information with its Street View mapping cars between May 2007 and May 2010. The company admitted publicly in May 2010 that it had collected the data, which the Federal Communications Commission said was not a breach of US laws.

On Friday the ICO said it would "be taking a keen interest in [Google's] operations and will not hesitate to take action if further serious compliance issues come to its attention".

The watchdog added: "Based on a detailed investigation, including an analysis of the data Google has recorded, the ICO has concluded that the detriment caused to individuals by this breach fails to meet the level required to issue a monetary penalty."

A Google spokesman said: "We work hard to get privacy right at Google. But in this case we didn't, which is why we quickly tightened up our systems to address the issue. The project leaders never wanted this data, and didn't use it or even look at it.

"We co-operated fully with the ICO throughout its investigation, and having received its order this morning we are proceeding with our plan to delete the data."