Thousands of nurseries and children's centres have been told to tighten up their child safety measures after parents raised fears over abuse and neglect.
Nearly half of the 25,000 childcare groups and childminders inspected by Ofsted over 12 months must do more to keep children safe, the watchdog said.
The worst offenders were Tony Blair's flagship after-school clubs - 7 per cent were failed for their safety records.
The findings came in a report based on inspections conducted between April 2005 and March 2006.
Dorian Bradley, Ofsted's director of early years, said parents could be confident that "the vast majority" of childcare organisations were satisfactory.
"However, there is still some way to go to ensure that every child in every setting is safe and sound, all day, every day," he said.
Inspectors received 1,500 complaints over child safety and another 400 raising concerns about health, including issues of neglect. Allegations from parents included:
* A person living in a childminder's household or working on day-care premises who was not suitable to have unsupervised access to children.
* A child had been smacked or mistreated in another way.
* Childcare premises were untidy and could be hazardous, or were "insecure".
* Inadequate adult supervision, leading to accidents or bullying between children.
Parents also complained that some childcare staff had poor hygiene standards and "children's individual toiletingnappy-changing needs had been ignored".
Others said children were left out in the sun or exposed to health risks from pets or food to which they were allergic.
Ofsted criticised some childcarers who put children's health at risk by smoking.
Each year around 80 children are seriously injured while left in childcare.
Ofsted said this represented about one serious accident in every 14,000 childcare places.
Katrina Phillips, chief executive of the Child Accident Prevention Trust, said: "Accidents are a leading cause of death and serious injury for children. So it is encouraging that the vast majority of childcare providers are providing a safe environment for children."
The report showed a sharp rise in the proportion of nurseries, children's centres and childminders judged "inadequate" for childcare - up from 1 per cent last year to 4 per cent this year.
But Ofsted said this rise was largely because inspectors had deliberately "raised the bar" for what counts as "satisfactory" and now expected higher-quality care.
Inspectors visited 2,383 "out-of-school" childcare groups. But more than one in 10 of these after-school clubs - which have seen a major government-backed expansion in recent years - failed to provide adequate childcare.
And 7 per cent fell short of the basic standards for making sure children stay safe, the report said.
A spokesman for the Department for Education and Skills said: "This report shows that the vast majority of parents can be confident their children are kept safe and healthy in their chosen childcare setting.
"We encourage childcare providers to consider how they can make additional improvements and raise the quality of provision further."