Lockdown measures will remain in place until at least March 8, Boris Johnson indicated as he earmarked that date for the reopening of England's schools.
The Prime Minister confirmed that hopes of pupils returning to class after the February half-term have been abandoned.
The March date is based on progress in vaccinating the most vulnerable groups in society by mid-February and then giving the jab time to take effect.
Mr Johnson also set out tougher measures to prevent the arrival of new strains of coronavirus into the UK, confirming plans for a 10-day quarantine in hotels or other government-provided accommodation for travellers from high-risk countries.
In a Commons statement after the UK's death toll passed 100,000, he said he would set out the Government's strategy for the "gradual and phased" easing of lockdown in the week beginning February 22.
But with reopening schools the Government's top priority, it seems unlikely that other lockdown restrictions will be eased before classes return.
England's schools are currently closed to all but vulnerable pupils and the children of key workers.
The Prime Minister told MPs: "The first sign of normality beginning to return should be pupils going back to their classrooms."
But he said the UK remains in a "perilous situation, with more than 37,000 patients now in hospital with Covid – almost double the peak of the first wave".
By mid-February much more will be known about the effect of vaccines in preventing hospital admissions and deaths, he said.
In the week commencing February 22, the Government will "publish our plan for taking the country out of lockdown".
"That plan will, of course, depend on the continued success of our vaccination programme, the capacity of the NHS, and on deaths falling at the pace we would expect as more people are inoculated," Mr Johnson added.
The Prime Minister said if schools reopen on March 8 the "economic and social restrictions" could be eased "then or thereafter".
The announcement on quarantine covers countries which are already subject to a travel ban due to concern over mutant strains of coronavirus, including South Africa, Portugal and South American nations.
"In order to reduce the risk posed by UK nationals and residents returning home from these countries, I can announce that we will require all such arrivals who cannot be refused entry to isolate in Government provided accommodation, such as hotels, for 10 days without exception," he said.
"They will be met at the airport and transported directly into quarantine."
Mr Johnson said the Government had been in contact with the devolved administrations on the issue so that "where possible we continue with a UK-wide approach".
Mr Johnson faced questions from Labour leader Sir Keir Starmer about the decisions which had led to the UK's high death toll.
"Why has the United Kingdom the highest number of deaths in Europe, why has the United Kingdom a death rate that is higher than almost anywhere in the world," he asked at Prime Minister's Questions.
Mr Johnson insisted it was not yet time for an inquiry into the handling of the pandemic "when we are in the throes of fighting this wave of the new variant, when 37,000 people are struggling with Covid in our hospitals".
He told MPs "there are no easy answers – perpetual lockdown is no answer" to the challenges posed by the virus.
In other developments:
– The Archbishops of Canterbury and York urged people to take a moment each day to pause and remember the more than 100,000 people across the UK who have died after contracting Covid-19.
– Bereavement support charities called for more help, asking for the £500 million allocated to mental health in England to be used to support those who have lost loved ones during the pandemic.
– Professor Neil Ferguson, a member of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said that if the Government had acted earlier and with "greater stringency" in September a lot of recent deaths could have been avoided.
– AstraZeneca chief executive Pascal Soriot predicted the UK will have vaccinated "maybe 28 or 30 million people" by March and will hit the target to administer jabs to the top four priority groups by mid-February.
– Professor Calum Semple, who sits on the Government's Scientific Advisory Group for Emergencies (Sage), said there could be another 50,000 deaths from coronavirus before the pandemic "burns out".
– Dr Richard Harling, director of health and care at Staffordshire County Council, said local authorities will be dealing with the virus "throughout the 2020s, into the 2030s".