UK Government ministers were once again under fire last night over data security when a series of embarrassing blunders emerged involving the loss of confidential personal information.

First, a police inquiry was launched after it was disclosed that two computer discs with details of more than 7000 motorists in Northern Ireland had gone astray in the post.

Secondly, it was revealed how confidential personal details of dozens of prisoners intended for Norfolk Police were wrongly delivered to a private company.

Finally, trades unions on Merseyside revealed that personal details of 1800 health authority staff had been accidentally sent out to a number of private firms.

The latest security breaches follow the loss by HM Revenue and Customs of two computer discs containing the child benefit database with details of 25 million people.

The discs with motorists' details went missing after they were sent by Parcelforce from the Northern Ireland Driver and Vehicle Agency (DVA) in Coleraine, County Londonderry, to the Driver and Vehicle Licensing Agency in Swansea.

According to a leaked letter obtained by Patsy McGlone who chairs the Northern Ireland Assembly Environment Committee, the details went astray at a sorting centre in Coventry.

The discs, which were not encrypted, contained the names and addresses of the registered keepers of 7685 vehicles together with the vehicles' registration mark, chassis number, colour and make.

The Northern Ireland Department of the Environment, which runs the DVA, explained that letters had been sent out to the motorists affected, warning them what had happened and that a helpline had been set up.

Arlene Foster, Northern Ireland's Environment Minister, confirmed a full-scale police investigation was under way after the DVA management admitted the blunder. Brendan Magee, DVA chief executive, said: "We have to hold our hands up, it should have been encrypted.

"Our own internal review is under way in relation to the HMRC that has been identified. Unfortunately, it was a week too late."

Officials realised last week that the data, sent on November 20 and 21, had gone missing.

In Norfolk, police said that letters containing confidential details of dozens of prisoners had accidentally ended up with a private company in the county. Police sources said that the letters included names, dates of birth and criminal records of inmates who were about to be released as well as the addresses they would be moving to when they left jail.

On Merseyside, Unite was demanding an inquiry after the names, dates of birth, and salary and pension details of 1800 employees of the Sefton Primary Care Trust were sent out to four private companies. According to the union, the firms were all bidding for a contract with the trust's sexual health department.

Last night, Theresa Villiers, Westminster's Shadow Transport Secretary, said: "It beggars belief that Labour is still losing CDs with thousands of people's personal data in the post. It looks like it has failed to learn anything from the HMRC catastrophe."

Alistair Carmichael for the Liberal Democrats said the loss of the DVA discs would have implications for the UK Government's planned identity card scheme.