A quarter of a million adults suspected a child was being abused in the past decade but did nothing about it, a children's charity poll indicates.
Many adults are confused about how best to deal with child abuse
A survey of 10,000 people carried out for the NSPCC found one in nine adults had suspected a child was being abused.
And 6% said they had suspected abuse but done nothing about it - equivalent to more than 250,000 adults in the UK.
Physical abuse was the most common worry, followed by neglect, emotional abuse, bullying and sexual abuse.
Nearly a third of suspected abuse cases were in the last 12 months.
Many adults who did take action were unsure what was the right thing to do, what would happen next and the consequences for the child, the research found.
But just over three quarters of those who acted and knew what had happened as a result said they felt it had made things better for the child.
NSPCC director Mary Marsh said: "In the next 10 years, millions of adults may come across children they are worried about.
"In fact, it is likely that each of us will either know a child we suspect is being abused or know someone who does."
Next month the NSPCC's Full Stop campaign will send information packs to nine million households containing advice on how to deal with child abuse.
Ms Marsh said these would help people spot the signs of abuse, to feel able to talk about it and work out what to do.
"Child abuse thrives on secrecy. Children can find it extremely hard to talk about their abuse.
"It is often left to adults to pick up the signs and share their concerns. People should trust their judgement."
She said people who suspected a child was being abused should talk their fears through calmly with someone they trusted before deciding what to do.
"Protecting children needs warm hearts and cool heads," she added.