HM Revenue and Customs (HMRC) has made a series of mistakes as it tries to apologise to parents for the loss of their personal details in the post, the Tories disclosed.
Conservative MPs have been contacted by parents concerned that HMRC - already reeling from the lost benefits discs fiasco - has been sending them the details of other parents.
It is the latest embarrassing revelation for the Government following the loss of the entire child benefit database of 25 million people.
It was contained on two computer discs which went missing in transit between HMRC offices in Tyne and Wear and the National Audit Office in London.
HMRC chairman Paul Gray resigned over the debacle last week, although the Government maintains the error was down to a junior official.
Manjit Mojaria, of Warwickshire, said she had received seven apologies from the HMRC intended for other people and containing their National Insurance numbers, child benefit numbers, names and addresses.
"It really is a very silly and absolutely awful mistake to make when trying to re-instil confidence," she said.
When she informed HMRC of the mistake, Ms Mojaria said she was given the impression she was "not in the minority".
Another incident involved a woman in Banbury, Oxfordshire, being sent a letter containing National Insurance and child benefit numbers that were not hers. She said: "Not only did they cock up the missing discs, they are also sending out families personal data to other families."
Shadow chancellor George Osborne said: "The HMRC can't even apologise without breaking people's confidences. Many cases are emerging all over the country of letters sent to parents to apologise for the loss of data, including the personal details and national insurance numbers of complete strangers."
