Missing: 25m people's personal data

He said the details included names, addresses, dates of birth, Child Benefit numbers, National Insurance numbers and bank or building society account details.

Paul Gray, chairman of her Majesty's Revenue and Customs (HMRC), which lost the discs containing the Government's entire Child Benefit database, has resigned over the affair.

The staggering scale of the loss means information on senior politicians, police officers and leading industrialists will be included in the missing data, which contains records on nearly half the UK's 60.5 million population. MPs gasped as Mr Darling revealed the scale of the loss in an emergency statement to the Commons.

The Metropolitan Police is now leading the hunt for the two password-protected discs and trying to discover how they went astray in transit from benefit headquarters in Newcastle to the National Audit Office (NAO) in London.

Mr Darling said they should not even have been sent in the first place, as a junior official breached all Her Majesty's Revenue and Customs standing procedures by transferring them via couriers TNT to the NAO.

The Chancellor stressed there was no evidence that they had fallen into criminal hands and said the public would be protected against any fraud by the Banking Code.

He told MPs: "The missing information contains details of all Child Benefit recipients: records for 25 million individuals and 7.25 million families. These records include the recipient and their children's names, addresses and dates of birth. It includes Child Benefit numbers, National Insurance numbers and, where relevant, bank or building society account details."

That effectively means the personal details of virtually every family in the country with a child under 16 have gone missing. Child benefit can be paid up to the age of 20 if the teenagers are studying for A-levels or on an approved training scheme.

The HMRC official who sent the CDs did not tell senior officials about the loss because they assumed the package was delayed, the HMRC said. The official believed the package was delayed by the postal strike or the NAO's office move and "hoped that it would turn up" and so kept quiet, an HMRC spokeswoman said.