YouTube has changed the way its autofill feature works after reports that some word combinations brought up paedophilic phrases.
Over the weekend many people reported that typing "how to have" would be completed with several variations on "s*x with your kids".
Other searches produced responses that also used the asterisked "s*x" word.
Some speculated that an attempt to troll YouTube results was responsible for making the phrases appear.
"Earlier today our teams were alerted to this profoundly disturbing autocomplete result and we worked to quickly remove it as soon as we were made aware," said a YouTube spokeswoman.
"We are investigating this matter to determine what was behind the appearance of this autocompletion," she added.
YouTube has not yet given an explanation of why the phrase "how to have s*x with your kids" was suddenly being so widely suggested.
Tony Stower, a policy and public affairs manager from the NSPCC, said YouTube should have done better.
"Social networks used by millions of children should never suggest dangerous or illegal content," he said. "It is not good enough for problems like this to go unaddressed until media coverage brings it to the attention of sites like YouTube."
Mr Stower said an amendment to a forthcoming UK bill covering data protection could mean all social networks have to put in place systems that try to keep people safe.
He said the NSPCC was urging politicians to back the bill when it comes up for a vote.
Charlie Warzel, writing on Buzzfeed, suggested that the use of the asterisk replacing the "e" in the word "sex" across lots of different searches suggested deliberate action was behind it.
"The results are very specific and could be the result of a co-ordinated campaign to game the algorithm," he said.
This could have been an attempt to populate YouTube with search results that embarrass the site, he said.
None of the videos that the "how to have" results linked to showed abuse of children.
The disturbing results came after a week in which YouTube was widely criticised for not doing enough to stop sexual predators targeting young users of the site.
Big brands including Mars, Lidl and Adidas pulled adverts from YouTube after investigations by the BBC and The Times found tens of thousands of predatory accounts being used to leave explicit comments on children's videos.