Schools may have to remain shut in order to control coronavirus transmission, senior scientists have warned.
Professor Neil Ferguson, a member of the Government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said there had been a "balancing act" since lockdown was initially eased between keeping control of the virus and maintaining "some semblance of normal society".
But he said planned school reopenings from next week may have to be postponed.
He told BBC Radio 4's World At One programme: "Clearly nobody wants to keep schools shut. But if that's the only alternative to having exponentially growing numbers of hospitalisations, that may be required at least for a period.
"There are no easy solutions here. My real concern is that even if universities, schools, do have staggered returns or even stay closed, how easy it would be to maintain control of the virus is unclear now, given how much more transmissible this variant is."
Earlier, Scientific Advisory Group for Emergencies (Sage) members Professor Andrew Hayward and Dr Mike Tildesley signalled the possibility of a "slight delay" to having pupils back on site, with latest figures from NHS England on Tuesday afternoon showing that a further 365 people who tested positive for Covid-19 had died, taking the total number of confirmed reported deaths in hospitals in England to 49,225.
The Government said it is "still planning for a staggered opening of schools" after Christmas but is keeping the plan under constant review.
Prime Minister Boris Johnson's official spokesman told a Westminster briefing: "We're still planning for a staggered opening of schools and we are working to ensure testing is in place.
"As we have said throughout the pandemic, we obviously keep all measures under constant review."
Earlier this month, the Government said exam-year students in England would go back to school as normal after the Christmas holidays, from January 4, but the majority of secondary school pupils would start the term online to allow headteachers to roll out mass testing of children and staff.
Schools in Wales, Scotland and Northern Ireland will also use staggered returns for pupils in January, with some pupils participating in online classes before the gradual reintroduction of face-to-face teaching later in the month or in February for some age groups.
Conservative MP Robert Halfon, chairman of the Commons Education Select Committee, said he "hope(d) very much" that schools would reopen from Monday, but called on Mr Johnson to "set out a long-term plan for education" and end confusion about the future of schooling during the pandemic.
He told Good Morning Britain: "I would welcome a statement either from the Prime Minister or the chief medical officer as to what the scientific evidence is, and also to set out a long-term plan for education – a route map out of this – because we can't have schools as a revolving door, with parents, the teaching profession and support staff not knowing from one day to the next what is going to happen."
The Government said soldiers would be drafted in to help schools set up testing facilities in order to welcome pupils back on site, but that prompted calls from unions for better support.
Mary Bousted and Kevin Courtney, the joint general secretaries of the National Education Union (NEU), have also written to Education Secretary Gavin Williamson, as well as Mr Johnson, reiterating calls for schools and colleges to remain closed for at least the first two weeks of January, except for vulnerable children and the children of key workers.
Steve Chalke, founder of Oasis, one of the largest multi-academy trusts in England, also suggested delayed reopening.
He told the Radio 4 Today programme: "We would suggest a week or two's delay to think it through, to do it well – and we think that if you really care about kids you would do this well – to invest now, to give time now makes sense."
Chris Foley, headteacher at St Monica's RC High School in Prestwich, Manchester, said it is a "huge challenge getting the balance right between supporting pupils' wellbeing and reducing community transmission".
He told the PA news agency: "We do feel that we want our school to be open, and we are equally concerned by the impact of uncertainty on the pupils.
"We have wonderful Year 11 pupils who just want to get on with their studies, take their exams and then move on to the next stage of their life.
"The disconnect between national policy and then the delivery of policy directives at a school level has been the most challenging part of this, to be honest."