Nurseries need to take a role in toilet-training youngsters because children are spending more time in childcare, the head of Ofsted has said.
Parents cannot potty train young children in a weekend, meaning that teaching the vital life skill is becoming a "co-operative exercise" between mothers and fathers and the childcare workers who look after the youngsters during the week, according to Amanda Spielman.
She suggested that nurseries are having to step up and help as children are being toilet-trained at a later age, and more are in some form of early education.
In a speech at the Pre-School Learning Alliance's annual conference in London, the Ofsted chief inspector also said it was "alarming" that more schools were reporting that children were turning up on the first day of their reception year (aged four or five), unable to use a toilet.
Speaking to reporters afterwards, she said that "the age at which children are potty trained has got a lot older in recent decades", adding that on one side, toilet training was happening at a later stage and at the other "more children are in sort of some kind of formal pre-school provision".
"So something that you used to be able to say was clearly, definitely, parents, and done before they got into education provision, now theres a sort of blurring of who, what, when."
Ms Spielman added: "So without taking responsibility away from parents, but if your child is in nursery every day, and toilet training is not something you can do in two days over the weekend, and say boom it is done, so in practice it becomes something that's got to be a bit of a co-operative exercise."
In her speech, the Ofsted boss said that being able to use a toilet was a "simple, but necessary, expectation" of four-year-olds.
"So it is alarming that more and more schools report children turning up on their first day of reception unable to do this.
"You have seen, there have been recent news stories about children being sent to school in nappies," she told delegates.
"Of course there are rare health complications that can be a good reason for this," she went on to say.
"But these exceptions aside, no-one could say this is an acceptable situation.
"It may not be common, but when it does happen it is disruptive for teachers, disruptive to other children's learning, and worst of all, can have a terribly negative effect on the child.
"Again, beginning school so far behind their peers and at risk of being teased."
Ms Spielman also said: "While parents clearly have the most important role here, it follows that nurseries and childminders must also play their part.
"After all, many pre-schoolers spend much of their daytime in childcare.
"What you do to identify children who are struggling, work with parents and monitor progress, can make a world of difference.
"I am not suggesting nurseries are substitute parents. Nor do I think children should start reception as perfectly turned out mini adults, who always go to the toilet unaided and never have accidents.
"But we know that the best nurseries work closely with families, help to establish simple routines, such as sleep time and potty training, as well as introducing children to foods that they may refuse at home."
Ms Spielman also used her speech to warn that young children must be allowed to take risks.
She told delegates: "Of course we expect you to take risk seriously and to supervise children properly.
"But equally, don't take away the climbing frame in case someone falls, or avoid journeys to the park for fear of crossing the road.
"Some level of risk is part of a proper childhood. And without it, we stifle childrens natural inquisitiveness and their opportunities to learn."
Neil Leitch, chief executive of the Pre-School Learning Alliance, said: "Childcare providers are already working incredibly hard to support children in the first few years of their lives, supporting families and children as they prepare to start school.
"And this won't change. But the fact is that three quarters of providers (74%) have told us, in Alliance research released today, that they have been regularly stressed about an issue relating to their work.
"It is also important to let children be children. The best childcare providers know that each child is unique and develops at their own pace.
"They know that the focus of the early years should be on ensuring that schools are ready for children, and not the other way around."