More children are starting school before they are fully toilet trained, teachers say.
A snapshot survey suggests many primary school staff are noticing a rise in the number of children wetting or soiling themselves.
The online survey of 848 staff for the Association of Teachers and Lecturers suggested the main cause was a lack of toilet training.
ATL said the problem put extra pressure on school staff.
Two-thirds of the respondents to the survey, carried out by the ATL with the charity Education and Resources for Improving Childhood Continence, said they had noticed an increase in children having toileting accidents during the school day.
It also found that 38% of respondents were in schools with no written policy for dealing with such issues.
ATL general secretary Mary Bousted said: "Schools need to give staff clear guidance on how to deal with toileting accidents so that they know what they are allowed to do, and who should be dealing with an incident.
"It is also important that education staff feel that have support from their school nurse or head, and that they know where to obtain guidance should they need it."
A foundation stage teacher from Wales stated: "We used not to take children until they were fully toilet trained.
"Now we accept them anyway as we operate in a deprived area and attendance at nursery is usually deemed to be in the child's best interest."
Another foundation stage teacher said: "This is a major problem for us - over 45% of our nursery children are not toilet trained when coming into nursery when they are three years old.
"We also have children who soil and wet a great deal, even in reception.
"Our parents just have no idea when and how to toilet train their children. We are having to put on a workshop to support them."
Children normally start school in the academic year that they turn five, but increasing numbers are attending pre-schools from the age of three.
Jenny Perez, director of ERIC, said: "Schools should be clear about their expectation that children should be using the toilet independently when they start school.
"They can support parents to achieve this by providing resources and information at the time the child's school place is confirmed."