The Information Commissioner is to investigate a fresh security lapse by the taxman as the confidential details of millions of parents were included in apology letters sent to their last known address.

National Insurance and Child Benefit numbers, vital for identity thieves, are dropping on to wrong doormats all over Britain as the fiasco over missing tax data deepens.

Codes belonging to seven strangers were posted to one mother by the taxman along with a letter apologising for breaching her privacy.

The Times reported yesterday that the mass maildrop was a gift to fraudsters who can use any stray, stolen or discarded letters to open bank accounts, claim benefits or get passports.

Q&A: how can I protect myself?

Advice for anyone who fears their personal data may be misused by fraudsters after Britain's worst ever security breach

• Government under pressure over taxman's giant blunder

Related Internet Links

• Online security: full coverage

Background

• Police step in as other CDs have gone astray

• Moment’s blunder puts half the country at risk

• Data watchdog seeks dawn-raid powers

• New data law 'urgently needed'

A spokeswoman for Richard Thomas, the Information Commissioner, said the mailshot would be included in the investigation he launched into the loss of 25 million people’s data last week. Sources at his office said: “This would be something that we would have looked at further, even without the original problem.”

The Information Commissioner is now pursuing three inquiries into breaches of confidentiality by HM Revenue & Customs as concerns grow that the Government shows insufficient care to personal data.

Nigel Evans MP, chairman of the All-Party Parliamentary Group on Identity Fraud, welcomed the widening of the formal probe.

“I hope the Commissioner will issue recommendations to government departments about what personal information it is appropriate to include on letters to the public,” he said.

“I hope he will urge the Government to include some kind of identity warning about what people need to do with such letters: shredding them or filing them away securely.”

Mr Evans has tabled a Commons question demanding to know the cost of the mass apology. The letters were sent second class but he feared the administrative bill would reach many millions of pounds. “It shows how slap happy they are about personal data,” he said. “It’s staggering.”

Every parent who fails to receive the letter of apology is being urged to ring a helpline manned by HMRC in case their private details have gone astray. One in ten Britons moves house each year, according to Government figures, but HM Revenue & Customs are blaming families over the new blunder for failing to notify changes of address.

A spokesman declined to break down the cost of the mailshot. Antifraud experts and police sources have urged the public to shred the messages, but HMRC said: “We can’t tell people how to dispose of their mail.”

The Conservatives blamed Alastair Darling for the latest fiasco.

George Osborne, the Shadow Chancellor, said: “The HMRC can’t even apologise without breaking people’s confidences.”

Times Recommends

• Third of Muslim students back killings
• Curfew tames feral yobs of Cornwall
• £3,000-a-month drug free abroad