Parents should “absolutely” send their children in to primary schools in areas where they’re open next week, the prime minister has said.

However, interviewed on the Andrew Marr show this morning, Boris Johnson repeatedly refused to say whether he would penalise local authorities that urge their schools to stay closed to most pupils in conflict with government guidance.

The government announced on Wednesday that most primary schools will reopen next week, while the start of term for secondaries was pushed back to January 11 for exam pupils, and January 18 for all others.

Primaries in Covid hotspot areas will remain closed for two weeks. The additional measures are meant to help combat rising case numbers and the spread of a new variant of Covid-19.

However, unions have called for primary schools across the whole country to move to remote learning for two weeks amid concerns over pupil and staff safety, with leadership unions NAHT and ASCL beginning legal action over the government’s plans.

Asked whether parents of primary school-aged pupils should send their children in next week, Johnson said: “Yes absolutely they should, in the areas where schools are open.”

“I would advise all parents thinking about what to do – look at where your area is. Overwhelmingly, you’ll be in a part of the country where primary schools tomorrow will be open”

It comes after minutes published by the government’s Scientific Advisory Group for Emergencies (Sage) show the group advised ministers on December 22 that it was “highly unlikely that measures with stringency and adherence in line with the measures in England in November [with schools open] would be sufficient to maintain R below 1 in the presence of the new variant”.

Sir Mark Walport, the government’s former chief scientific adviser and a member of SAGE, told Marr earlier that the new Covid variant was transmitting “more readily” in young people and warned it would be “very difficult” to keep under control without “much tigher” measures, which could “potentially” include school closures.

But speaking this morning, Johnson claimed schools were “safe” and that the risk to both young people and staff is “very small”.

Pressed on the SAGE advice, Johnson said: “Well actually what they said was that we needed to take tougher measures.

“Sir Mark Walport, who you just spoke to, agrees. And I agree with that. I think the issue really we’ve got to work out… The evidence is not clear, because we are looking at tier 4, and what happens in tier 4 areas, and we need to see whether those extra steps that we’ve all taken in tier 4 areas are going to work in driving the virus down.”

Asked whether primary schools in other areas of the country would close if the evidence showed tier 4 restrictions were not working, Johnson said the situation would be kept “under constant review, but we will be driven not by any political considerations but entirely by the public health question”.

The PM also refused to say if the government would take legal action against councils in areas not affected by primary school closures that have asked them to stay closed. It comes after ministers took action against Greenwich Council in December after the authority urged its schools to close early for Christmas to help stop the spread.

Brighton and Hove Council yesterday urged primary schools in its area  to open remotely from next week, despite not being on the list of areas where government has told primaries to stay shut.

“We’ll work very hard with authorities across the country to get our message across, that we think schools are safe, that schools are safe,” Johnson said.

He added: “My message to such councils is that they should be guided by the public health advice, which at the moment is that schools are safe in those areas where we’re not being driven by the new variant to close them, and that the priority has got to be children’s education, but obviously we want to work with them.”

It comes as Amanda Spielman, the chief inspector of schools, intervened in the debate over school closures to welcome the fact that children will resume lessons next week, albeit with some learning online, and warned that we “cannot furlough young people’s learning or their wider development”.

Writing in the Telegraph, Spielman said there was a “real consensus that schools should be the last places to close and the first to re-open, and having argued for this since last spring, I welcome it”.

“Because it is increasingly clear that children’s lives can’t just be put on hold while we wait for vaccination programmes to take effect, and for waves of infection to subside.”

In a statement issued this morning, Dr Susan Hopkins, senior medical adviser at Public Health England, acknowledged that school closure “can reduce transmission”, but said the public health advice “remains that they should be the last to close and the first to re-open”.

“Where rates are extremely high, continuing to rise and the NHS is under significant pressure, it may be necessary to move to remote learning as a last resort.

“The majority of children and young people have no symptoms or very mild illness, but we all need to take responsibility for driving infections down if we want to keep schools open for our children – we must continue to reduce our contacts, keep our distance washing our hands and wearing a mask to help stop the spread of the virus.”