Boris Johnson has said he wants schools in England to reopen as planned at the start of January, but he said measures were being kept “under constant review”.

The Prime Minister told a Downing Street press conference that the Government wants secondary school pupils to return to face-to-face lessons in a staggered way in the new year if they “possibly can”.

But Mr Johnson said the “commonsensical thing to do” was to follow the path of the epidemic.

His comments came after Britain’s largest teachers’ union called on the Government to allow schools to move classes online for most pupils for a fortnight in January to allow Covid-19 cases to fall.

The leaders of the National Education Union (NEU) have written to Prime Minister Boris Johnson and Education Secretary Gavin Williamson to call for all students to be kept at home – except key workers’ children and vulnerable pupils – over the first two weeks of January to get mass testing set up.

Last week, the Government announced that most secondary school and college pupils’ return to class in England would be staggered in the first week of January to help schools roll out mass testing of students.

But scientists have suggested that the mutated coronavirus strain could more easily infect children.

When asked whether he could guarantee that schools would be back on the planned start dates, Mr Johnson said: “The most useful thing I can tell you at this stage is obviously we want, if we possibly can, to get schools back in a staggered way at the beginning of January in the way that we have set out.

“But obviously … the commonsensical thing to do is to follow the path of the epidemic and, as we showed last Saturday, to keep things under constant review.

“It is very, very important to get kids and keep kids in education if you possibly can.”

On Sunday, Health Secretary Matt Hancock refused to confirm that schools in Tier 4 areas would reopen in January if cases continue to rise.

Asked about schools reopening in Tier 4 on Monday, the PM’s official spokesman said: “No change … it’s rightly been a national priority for all pupils to return to school full time.”

In a letter to Mr Johnson and Mr Williamson, Mary Bousted and Kevin Courtney, joint general secretaries of the NEU, voiced concerns about reports that the new variant of Covid-19 has increased transmissibility.

It says: “Firstly, we believe that you should allow and encourage heads in ensuring that first two weeks of learning should be online, apart from key worker and vulnerable children, to allow cases to fall further and to allow time to properly set up the system of mass testing.”

The letter adds: “Secondly, we believe that you should ask the local directors of public health to set the system of mass testing.

“We believe that the Government could support them via a national advertising campaign to find the staff and volunteers needed, as you did in finding the volunteers to help the NHS at an earlier stage.

“We would hope that such a system could then test all children, at their school site, prior to a return to in-person teaching from January 18.”