Boris Johnson has said parents should send primary-age children back to schools that are open this week, as he hinted at tougher coronavirus measures for England.

The Prime Minister said he has “no doubt” that classrooms are safe and that the risk to young people was “very, very small” amid calls from teaching unions to close all schools for the next two weeks.

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Referring to the tiers system, Mr Johnson said that coronavirus restrictions in England are "probably about to get tougher" due to rising infection rates.

Appearing on the BBC's Andrew Marr Show this morning (Sunday), the Prime Minister said: "Schools are safe. It is very, very important to stress that.

"I would advise all parents thinking about want 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."

Meanwhile, Mr Johnson signalled that despite vaccines "coming down the track in their tens of millions", regional restrictions in England were likely to get tougher.

"What we are doing now is using the tiering system, which is a very tough system... and, alas, probably about to get tougher to keep things under control," he said.

It comes as the NHS ramps up its vaccination programme with the newly approved Oxford University and AstraZeneca jab, with 530,000 doses available for rollout across the UK from Monday.

Labour leader Sir Keir Starmer said the vaccine was "our great hope", adding: "I want the Government to throw everything it can at this, harnessing the extraordinary talents of our NHS so we can be vaccinating at least two million Brits a week by the end of the month."

But, writing in the Sunday Mirror, he criticised "a chaotic last-minute U-turn on schools", adding: "Confusion reigns among parents, teachers and pupils over who will be back in school tomorrow and who won't."

On Friday, education secretary Gavin Williamson confirmed that all of London's primary schools would remain shut to most students, rather than just those in certain boroughs as set out earlier in the week.

Primary schools in the capital and some surrounding areas are not due to reopen until January 18, but elsewhere, including Bucks, students are expected to return to classrooms on Monday.

Asked whether he could guarantee schools will reopen on January 18, Mr Johnson added: "Well, obviously, we're going to continue to assess the impact of the Tier 4 measures, the Tier 3 measures."

International Trade Secretary Liz Truss was more confident on the matter.

Pressed on whether the majority of secondary schools would open by January 11 and 18, depending on the area they are in, Ms Truss told Times Radio: "Absolutely. That's what we are seeking to do.

"I'm a parent of secondary school children myself, so I know the challenges of making sure your children are keeping in touch with online learning, and we want to get those schools open."

Newly confirmed coronavirus cases were higher than 50,000 for the fifth day in a row when UK figures were released on Saturday, with a record-high of 57,725 lab-confirmed cases and another 445 deaths within 28 days of a positive test.

Professor Sir Mark Walport, who is a member of the Scientific Advisory Group for Emergencies (Sage), warned that it would be difficult to keep the new variant under control without "much tighter" social distancing measures.

Asked if this included closing schools, the former chief scientific adviser told the BBC's Andrew Marr Show: "We know that transmission occurs within schools.

"We know that a person between 12 and 16 is seven times more likely than others in a household to bring the infection into a household.

"And we know that there was a small dip in the amount of transmission in school children after the half term, which then went up again when they went back."

On Saturday evening, the Department for Education said remote learning was "a last resort" and classrooms should reopen "wherever possible" with appropriate safety measures to help mitigate the risk of transmission.

Ofsted chief inspector Amanda Spielman, writing in the Sunday Telegraph, said children's education cannot be put on "furlough" and that school closures should be kept to a minimum.

But general secretary of the National Education Union (NEU) Dr Mary Bousted earlier said schools should stay closed for two weeks to "break the chain" of transmission and prevent the NHS becoming "overwhelmed".

The union, which represents the majority of teachers, has advised its members it is not safe to return to classrooms on Monday.

NAHT general secretary Paul Whiteman said the union had started preliminary steps in legal proceedings against the Department for Education, asking it to share its scientific data about safety and transmission rates.