HM Revenue and Customs' lost 25m child benefit claimants' details.

The Ministry of Justice launched an investigation when the data disappeared after being sent by recorded delivery.

A spokeswoman refused to say whether the discs contained details of at least 55 defendants and other restricted data, as reported in the Daily Mail.

It follows a series of incidents of missing data the biggest being the loss of 25m people's child benefit details.

According to the Daily Mail, the four CDs were posted on 15 December and potentially included the names and addresses of alleged victims and witnesses.

Stolen laptops

A statement by Her Majesty's Inspectorate of Court Administration confirmed that four CD-Roms were missing.

"They were sent recorded delivery. Ministers and the information commissioner were notified immediately it was recognised that personal data had been lost," it said.

"An investigation is under way so it would be inappropriate to comment further at this stage."

The latest data disappearance is a further embarrassment to the government which has had to admit a series of large scale losses of personal data.

Defence Secretary Des Browne announced an inquiry into military security after he admitted a Royal Navy laptop stolen in October 2006 contained much of the same data that was on a computer taken from an official's car in Birmingham this month.

Missing driving test details

Data included passport, National Insurance and driver's licence numbers, family details and NHS numbers for about 153,000 people who applied to join the armed forces.

Banking details were also included for around 3,700 people.

But ministers were not told about the earlier theft of a Royal Navy laptop in Manchester, or of an army recruiting laptop in Edinburgh in 2005, until recently.

In October 2007, two child benefit discs containing the personal details of 25 million people went missing.

The HM Revenue and Customs discs containing the entire child benefit database, unregistered and unencrypted had been sent to the National Audit Office - but they did not arrive.

In December, Transport Secretary Ruth Kelly said the details of three million candidates for the driving theory test had disappeared.

Names, addresses and phone numbers - but no financial information - were among details on a computer hard drive which went missing in the US in May.

Cabinet Secretary Sir Gus O'Donnell placed a ban on civil servants taking unencrypted laptops or computer drives containing personal data out of the office.