The prime minister's suggestion that some children could start returning to schools in England from 1 June has been described as "reckless" by the largest teaching union.
In a pre-recorded address to the nation on Sunday, Boris Johnson said the start of next month was the earliest possible date to consider sending pupils back to class.
He said that by that time "we believe we may be in a position to begin the phased reopening of shops, and to get primary pupils back into schools, in stages, beginning with reception, Year 1 and Year 6".
The National Education Union says 85% of its 49,000 members who responded to a survey following Mr Johnson's speech disagreed with the suggestion to restart lessons for some year groups, while 92% said they would not feel safe with the proposed wider opening of schools.
Joint general secretary Dr Mary Bousted described the prime minister's announcement as "nothing short of reckless".
She told Sky News: "We're clear that the government hasn't answered fundamental questions about the safety of a return to school, around the potential spread of the virus in schools, and around basic safety precautions.
"We don't know the evidence on which Boris Johnson is saying it's safe to return to schools and I suspect neither does he."
More from Covid-19
Dr Bousted added: "We know that when schools are open, the evidence appears to be that they're highly effective in spreading the virus.
"So we have huge concerns for the pupils themselves going back to the schools, for the adults working in schools, and for the families and relatives and friends that these children will go back to."
She also suggested that the reason behind the move could be to benefit working parents.
"We don't understand the government's actions – the suspicion must be that it's harder to do work at home with very young children, and so there's an economic imperative to get those children back into school."
Dr Bousted claimed the prime minister's speech was "contradictory" and "unclear", adding: "I think frankly, it's a mess."
Chris Dyson, headteacher of Parklands Primary School in Leeds, told Sky News that "in an ideal world", reopening schools is what everyone wants, but that it "has to be safe for everyone".
He said: "Sadly, Boris Johnson said himself that he wasn't confident that the five tests he'd set in place had been passed anywhere near yet.
"So to think that we're going to be in a position to open up the school on 1 June for reception and Year 1 children is absolutely ridiculous, because these children will have no sense of social distancing whatsoever.