Parents have been shocked after children as young as nine were sent letters telling them they have to be tested for sexually transmitted diseases.
Harrow PCT has sent letters addressed to kids asking them to attend chlamydia screening sessions which are meant for 22-24 year olds.
Linda McDonald, of Ventor Avenue, Stanmore, was shocked when she opened a letter addressed to her 10-year-old son asking him to come along for a screening.
She said: "I am so angry. I couldn't believe it.
"My son gets so excited to open his own post so I saved it for him.
"It was only when I spoke to another parent at the school gates who told me her daughter had one that I opened it.
"The PCT told me it was human error but it is simply not got enough. There will be a lot of children who will go home and open it after school."
The letter offered young people a chance to win an iPod if they went along for a test and also asked the children to tell their "partners" to go for the screening.
A 34-year-old mother of three, of Shaftsbury Circle, in South Harrow, who did not want to be named, was flabbergasted the healthcare trust had made such a big mistake.
She said: "It's disgusting, words can't even describe.
"It was addressed to my nine-year-old, it was in a plain envelope, if she was at home she would have opened it. She is old enough to read sexually transmitted diseases, but she would have asked me what chlamydia was?
"I spoke to a receptionist and she gave me another number to ring but warned me I will have trouble getting through because there has been a high volume of calls as lots of parents have been complaining.
"If our name is on this list where else would it have been released to?"
A Harrow PCT spokesman said: "The intention was to send these letters to all young people aged 18 to 24 years.
"Unfortunately, due to an administrative error, a large number of letters were sent to children and young people of a younger age.
"The letters also contained the wrong telephone number, directing patients to Greenwich Primary Care Trust’s Chlamydia screening hotline. The correct number for enquiries is 020 8422 6644.
"We would sincerely like to apologise for any distress or confusion caused to families in Harrow. We will be writing to all those concerned to apologise directly."