Boris Johnson says parents have a “moral duty” to send their children back to school next week.
Schools in England and Wales are due to reopen in September after closures forced by coronavirus.
The prime minister has urged parents to bring their children back to school.
On Monday, his spokesman was asked if headteachers should fine parents who don’t send their children back.
He replied: “The PM’s words are clear. We believe there’s a moral duty.
“The prime minister’s clear, he believes that every child should be back in school next week.
“We’ve also been clear that the risk posed to students from COVID is less than the risk of them not attending school.”
The spokesman said fining parents was the “last resort”.
Earlier, the prime minister said in a series of tweets: “I have previously spoken about the moral duty to reopen schools to all pupils safely.
“We have always been guided by our scientific and medical experts, and we now know far more about coronavirus than we did earlier this year.”
Johnson said the risk of children contracting COVID-19 is “very small”, echoing the view of England’s deputy chief medical officer Dr Jenny Harries.
She said on Monday that children are more likely to get the flu or be involved in a road accident than they are to catch coronavirus at school.
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A Public Health England analysis, published on Sunday, found there were 67 single confirmed cases, four “co-primary cases” (two or more linked cases diagnosed at the same time) and 30 outbreaks of COVID-19 in schools during June.
It said the majority of cases linked to outbreaks were in staff.
Schools in Scotland reopened earlier this month, while those in Northern Ireland welcomed pupils again on Monday. English and Welsh schools will follow suit in September.
On Monday, Paul Whiteman, general secretary of school leaders’ union NAHT, said fining parents for not sending children back to school would not be helpful and would risk the relationships teachers have with families.
He told BBC Breakfast: “We can engage with those that still have a lack of confidence hopefully without fines.”
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