Britain's tax and customs service lost banking data and personal details of 25 million people - almost half the country's population - when two computer disks went missing in the post, Treasury chief Alistair Darling said Tuesday. Paul Gray, chairman of HM Revenue & Customs, has resigned over the error, which happened when officials sent disks to a government audit office through an internal postal service. Darling said the delivery was not being tracked and was missing for three weeks before any alarm was raised. Darling said the disks contained details of the 7.25 million families in Britain claiming child benefit - a tax-free monthly payment available to everyone with children. The figure represents almost half the families in Britain, and the majority of the country's children. Britain's population is about 60 million. The information on the disks included parents' and children's names, addresses, dates of birth, national insurance numbers and banking details. "I regard this as an extremely serious failure," Darling told lawmakers in the House of Commons. He insisted there was no evidence the data had fallen into the hand of criminals and said police were involved in a hunt for the missing disks. He said banks had been told to look for signs of suspicious activity. Conservative Party home affairs spokesman George Osborne said the government needed to "get a grip and deliver a basic level of competence." He said the mistake meant that "the names, the addresses and the dates of birth of every child in the country are sitting on two computer discs that are apparently lost in the post, and the bank account details and national insurance numbers of 10 million parents, guardians and carers have gone missing." HM Revenue & Customs is responsible for collecting taxes, distributing some welfare payments and carrying out border checks. Michael Fallon, a fellow opposition Conservative lawmaker, said the incident heaped pressure on Darling, already contending with woes at the mortgage lender Northern Rock. The lender ran into trouble over its heavy reliance on short-term money markets for funding, prompting the first run on a British bank for almost a century. It has needed emergency government loans of an estimated 24 billion pounds ($49.2 billion) to stave off collapse. Being Treasury chief "is an enormous job and so far Mr. Darling has not looked very sure-footed," Fallon told British Broadcasting Corp. television.