Nearly 1,000 parents rang Harrow health bosses after their children received letters asking them to have an STD test, confidential documents reveal.
Many mums and dads were left thinking their children as young as seven had chlamydia, the secret NHS report said.
The inquiry into the blunder puts the blame on a single bureaucrat who put the wrong date of birth into a database.
But the document, released under the Freedom of Information Act, said the letter sent in October by Harrow Primary Care Trust hadn't been checked before it was sent to 10,381 homes.
From now all large mail shots sent by the trust will have to be checked by whoever's signature is at the bottom.
The letter sent by the trust on October 22 told children aged seven and up they could win a iPod if they agreed to have a chlamydia test.
It had been intended for 15 to 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 for a screening.
She told the Observer at the time: "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 letter also asked the children to tell their "partners" to go for the screening.
When parents rang to complain they found they had actually got in touch with Greenwich PCT in south-east London, whose number had been put on the letter by mistake.
The report said 500 parents "expressed dis-satisfaction" but said none had made a formal complaint.
Director of public health Dr Andrew Howe said: "I would like to sincerely apologise to the
families affected by this mistake, and assure them that, of course, Chlamydia screening is
only necessary for sexually active adults and young people, in line with the national
programme to offer Chlamydia screening to those aged 15 to 24."
A spokesman for the trust said the database operator had not been disciplined.
Chlamydia is the most common sexually transmitted infection, affecting around one in 10 sexually-active young people who are tested.
Almost three quarters of people with Chlamydia do not know they are infected, and are at risk
of passing it on every time they have sex.
Undiagnosed Chlamydia can lead to infertility in men and women, miscarriage and premature birth.