A Home Office radio commercial publicising a child protection website has been banned because it could have led people to pornographic material.
The Advertising Standards Authority (ASA) said the advert could not be broadcast again in its current form.
The website advertised, thinkuknow.co.uk, advises children on how to stay safe online.
But a listener who misspelt the address found a series of links led her to adult porn websites.
The advert said: "Giving out personal info could let a paedophile track you down. Be smart online, be safe online."
But instead of typing 'u' in the web address, the woman wrote 'you', taking her to a different website.
Unacceptable for broadcast
The Child Exploitation and Online Protection Centre (CEOP), which has now taken over the radio campaign, said the pornographic material was at least four clicks away from the misspelt web address.
It added users would be aware by that point that they were not accessing a child protection website.
But the ASA said the advert had not made it clear how the web address was spelt, and as a result ruled that the advert indirectly publicised adult material and sexual services.
The industry watchdog said it was concerning because "the ad was aimed at teenagers and the service being promoted was to help them stay safe online".
"Although we recognised that there was no intention, we considered that a significant effect of the ad had been to indirectly publicise services which were unacceptable for broadcast."
The Home Office has since apologised.
A spokeswoman for the government department said: "We will of course comply with the ASA's decision and apologise for any unwitting problems this may have caused by the similarity in name with this search engine."