The Scientific Advisory Group for Emergencies, the body which advises ministers on the COVID pandemic, delivered the stark new advice at a meeting last week, an adviser familiar with their conclusions said.

Led by the government's Chief Scientific Adviser Patrick Vallance, SAGE agreed that, because of the prevalence of a new strain of the virus identified in the U.K. in the autumn, the reproduction rate — known as R — would surge further above 1 in January even if the prime minister implements another November-style national lockdown in the New Year.

In other words, the experts' advice to Johnson was that a New Year national lockdown in England identical to one he ordered in November — during which schools remained open — would not be enough to keep the virus under control, and that even stronger measures would be needed.

SAGE's advice was that the R rate might be kept below 1 if schools stay closed in January. The advisory body found that closing secondary schools would have a bigger impact than shutting primary schools.

Last summer, the prime minister said it was "vitally important" that children go back to school, after many were taught virtually between March and the summer holidays. Earlier last week, however, Johnson told a Downing Street press conference the government would stick to the plan to stagger the reopening of schools "if we possibly can."

At the same SAGE meeting, the committee also expressed increasing alarm about a second new strain of COVID believed to have traveled to the U.K. from South Africa. The scientists told Johnson that the South African strain is spreading even more quickly than the new strain identified in Britain, and that its mutations raise concerns about immunity. This has become a major concern among government scientists, a Whitehall official said.

The Sunday Telegraph reported that Education Secretary Gavin Williamson had told allies that he faces an "enormous battle" to keep secondary schools open next month, in a story that appeared to attempt to prepare the public for an announcement this week. Officials at the Department for Education will hold a crunch meeting with Downing Street on Monday to decide whether schools can reopen, the paper reported.