Prime Minister Gordon Brown tried to reassure Britons their personal details were safe Wednesday after one of the biggest security breaches in the country's history left millions of people exposed to identity theft and bank fraud.

Two computer disks that went missing while being sent from one government department to another contained names, addresses, birth dates, national insurance numbers and — in some cases — banking details for 25 million people, nearly half the country's population.

The disks were password protected but the information on them was not encrypted, officials said.

"I profoundly regret and apologize for the inconvenience and worries that have been caused to millions of families that receive child benefits," Brown told the House of Commons. "We have a duty to do everything that we can to protect the public."

Brown said he had asked security experts to work with government departments to check their procedures. He said the information commissioner also would be given the power to carry out spot checks on government departments.

Brown said he stood by Treasury chief Alistair Darling, who revealed the lapse at Britain's tax and customs service. There were gasps from lawmakers when Darling described the scale of the loss on Tuesday.

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. He said the delivery was not being tracked and was missing for three weeks before any alarm was raised.

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, he said.

"The police tell me there is no evidence there has been any criminal or unusual activity," Darling said.

Darling, already rocked by fallout from the run on troubled mortgage lender Northern Rock, said he was not going to resign.

"I am not going to start running away from things when things get difficult," he told British Broadcasting Corp. radio. "It is difficult, it is unwelcome in every respect, and I am determined to see it through."

Information Commissioner Richard Thomas, the official charged with overseeing data protection, said the lapse was "a shocking case."

"I am at a loss to find out what happened in this situation. It is not just about the law. It is about retaining the trust and confidence of the population where so much information is entrusted to government," he told the BBC.