It is “inconceivable” that schools could reopen without some form of social distancing being put in place, England’s most senior working minister has said.

Dominic Raab, first secretary of state and foreign secretary, who is deputising for the prime minister Boris Johnson, said possible measures are currently being tested by scientists.

But he said he does not want to “worry” teachers by discussing the details at this stage.

Meanwhile, a teachers’ leader has warned that social distancing in schools will be “very, very difficult” to achieve and said that it is still premature to be talking about reopening them.

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Background: Social distancing in schools ‘impossible’

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Some teachers have already warned that social distancing in schools is “impossible”.

Coronavirus: When will schools reopen?

Mr Raab was asked this morning on the BBC One’s The Andrew Marr Show: “Is it possible for schools to come back without social distancing?”

In response, he said: “I think it is inconceivable they would come back without further measures as are already being applied in those schools that are open for key workers and looking at how that could be done.”

Mr Marr pressed him for more detail, suggesting that when schools open class sizes might have to be smaller and that pupils might attend at different times - for instance, for half-days or part of the week.

But Mr Raab refused to be drawn, saying: “If I start suggesting one or other thing now without us knowing where the virus is and without us testing these measures with the scientists, which is the homework we are doing, we send the wrong signal and we don’t provide reassurance - we actually might provide worry for parents and teachers.”

The foreign secretary added: “The most important thing is to look at the extent to which schools would be a vehicle for allowing the virus to spread.

“We know that among children and young people it is much less likely to have a dangerous impact on them but children as a means for spreading the virus between households is something where we need the scientists to give further evidence.

“So we will look at all those measures but I’m not going to be drawn on the detail because we might end up changing our mind once we know the fuller evidence.”

However, Scotland’s first minister, Nicola Sturgeon, who was also interviewed on the programme, was less guarded.

She confirmed that pupils attending on different days or for half-days “may well be what we have to put in place”.

However, she stressed that it would “not be right or safe to open schools right now and that might continue to be the case for some time to come.”

She added, however, that it was important to start talking about what the future might hold to make it “more likely we take the public with us”.

Last week Ms Sturgeon published a paper on how Scotland could come out of the lockdown, saying it was an attempt at “levelling with the public” and that it would “evolve into a detailed plan”.

The NEU teaching union told Tes today that social distancing would be inevitable at the point where schools reopened - but it would be “very, very difficult” to put in place and would have to be worked out in negotiation with unions on a “school-by-school basis”.

NEU general secretary Mary Bousted also stressed that it was premature to be talking about opening schools.

She said the rate of transmission of coronavirus would need to be reduced dramatically and widespread community testing and contact tracing would have to be in place before schools could open.

“The NEU is very clear neither of these criteria is anywhere near being met,” she told Tes. “The rate of transmission is still far too high and we can’t even meet testing targets for NHS staff, never mind community testing and contact tracing. That is going to take a huge logistical effort to set up.

“When we do go back, Dominic Raab is right - it is inconceivable we would go back without social distancing and how that will be done in schools needs to be a matter of consultation and negotiation with recognised unions.

“It will mean different things in different schools and will be set up in different ways. But it will be very, very difficult to do.

“The countries where there is social distancing in school like Denmark have a much better pupil-teacher ratio than England, and in Germany the school day is usually 9am to 1pm.

“The reaction of teachers here, seeing what’s happening in Denmark and Germany, is ‘I wish’ because even new schools have small classrooms with large numbers of pupils.”

An Australian study published today says children are unlikely to transmit Covid-19 between each other or to adults.

Australia plans to reopen classrooms on Friday and prime minister Scott Morrison has said the risk for school staff there is “in the staffroom” not the classroom.

However, the teaching unions in Australia remain concerned by the prospect of schools reopening, saying there is “little clarity about how governments are going to ensure a safe working environment for teachers, principals and support staff”.