Re-opening schools in March could bring the Covid R rate back to one, or above it, an expert has warned.
"We're not absolutely certain. It looks as if it would be touch and go," Professor John Edmunds told the Peston show, when asked if reopening schools could lead to an increase in the reproduction rate of coronavirus.
The epidemiologist from the London School of Hygiene and Tropical Medicine continued: "So if we opened up schools, I think the reproduction number would get close to one and possibly exceed one.
"If we opened them completely, if we opened secondary schools and primary schools both at the same time, I suspect we would be lucky to keep the reproduction number below one."
Speaking about the new Covid variant identified in Bristol, he said: "I don't know whether the Bristol variant is any more transmissible than the Kent variant. I suspect it isn't.
"Where it has an advantage it is may be able to infect people who were previously infected, or have been previously vaccinated. That's the worry with that particular virus."
Asked if he had a sense of what kind of time scale there would be for a phased return to normal, Prof Edmund said: "It's to do with how fast you vaccinate people. That's really the great limiting factor. The faster we vaccinate the high risk groups, the better, so the quicker we can ease up.
"Eventually, we're going to switch to vaccinating lower risk groups, and then we will really have an impact on transmission.
"At the moment, we're vaccinating the elderly, who don't really play a great role in transmission. They tend to be at the end of transmission chains, not in the middle."
He warned against easing restrictions too early, before enough people are vaccinated.
He told the Peston show: "Most people have just had a single dose, they're not fully protected. So I think easing up now would be really dangerous."
When ITV News' Political Editor Robert Peston suggested a scenario where high risk groups had completed the second dose of the vaccine, Prof Edmund said: "I still think you'll get a lot of cases in vaccine failures.
"Say the vaccine is 90% protective, let's say and that would be very good. We vaccinate 90% of the high risk people. Then if you multiple 0.9 by 0.9, you have 0.81 - so you've protected 81% of the population, leaving almost 20% who are not protected.
"And you'd have still, under that very optimistic scenario, you'd still have 20% of the highest risk group unprotected. So if you let it rip, they would get infected very rapidly and soon be filling your hospitals and eventually your morgues."
Asked about his views on summer holidays this year, he said: "I think we do have to keep our borders pretty tight at the moment. Nobody likes this, but we've identified these significant variants that are out there and we don't have new vaccines that could arm ourselves against these new variants yet."
He added: "I do think we need to be very cautious about travelling."