A letter was last night sent to West Yorkshire's Chief Constable asking what steps he is taking to safeguard families after sensitive personal details of every child benefit claimant in Britain went missing.

Shadow Home secretary David Davis is calling for Sir Norman Bettison to offer his assurances West Yorkshire Police are doing everything they can.

Earlier this week tens of thousands of Bradford families were told to watch their bank accounts after it emerged that two computer discs of child benefit data, mislaid by HM Revenue and Customs (HMRC) last month, contained files on 66,725 families and 128,610 children in Bradford.

The files included claimants' and children's names, addresses, birth dates, national insurance numbers and, in some cases, bank details.

Following the security blunder, which placed some 25 million individuals and 7.2 million families across Britain at risk of identity fraud, banks and building societies urged claimants to be "vigilant" in monitoring their accounts for suspicious activity.

The letter reads: "You will be aware of the potential catastrophic criminal consequences of the loss of 25 million personal and financial records by HMRC, revealed this week.

"Following this loss, I would be very grateful if you could inform me of what steps your force is taking to protect against criminal fraud involving both the financial and personal records of the families involved."

Prime Minister Gordon Brown this week apologised for the security lapse and said a junior official should never have been in a position to post the sensitive information.

He said there was no evidence the data had fallen into criminal hands. If fraud did occur customers would be reimbursed for any losses under the banking code.

A spokesman for West Yorkshire Police said the force was waiting to receive Mr Davis's letter.