Data collected in the approaching UK census may not be as confidential as the Office of National Statistics claims, according to a data protection expert.

The ONS, which operates the census, says on its website that the data will remain confidential for 100 years.

But Chris Pounder, director at data protection and privacy training provider Amberhawk, points out that the census data is in fact subject to the Statistics and Registration Service Act 2007.

This act contains a clause stating that the ONS may disclose data under a number of conditions, including disclosures that are "made for the purposes of a criminal investigation"; "made in pursuance of an order of a court"; or "made to an approved researcher”.

"The law permits the board [governing the ONS] to disclose data if it wants," Pounder told Information Age today. "The current board says no data will be disclosed, but the law says that it can be. What happens when the board members change?"

Pounder describes the clause in question as a hangover from the previous government’s "disdain for personal privacy". "It is not a clause to protect confidentiality," he wrote in a blog post over the weeknd. "It is a clause to remove that confidentiality."

Today, Pounder said he would like to see the ONS take the step made by the Department of Children, Schools and Families, which demanded the removal of a similar clause in 2009.

"I’d like to see confidentiality guaranteed in black and white," he said, adding that the Protection of Freedoms Bill, currently under debate by parliament, could be used as a vehicle to do just that.

"If there’s any mistrust about what the ONS might do with the census data, people will be reluctant to fill it out and that would defeat the whole object," Pounder remarked.