Prime Minister Boris Johnson said on Sunday tougher lockdown restrictions were probably on the way as COVID-19 cases keep rising, but that schools were safe and children should continue to attend where permitted.
COVID-19 cases in Britain are at record levels and the increase in numbers is fuelled by a new and more transmissible variant of the virus. The government has cancelled the planned reopening of schools in and around London but teaching unions want wider closures.
Much of England is already living under the toughest level of restriction under a four-tier system of regional regulations designed to stop the spread of the virus and protect the NHS.
But, asked in a BBC interview about concerns that the system may not be enough to bring the virus under control, Johnson said restrictions "alas, might be about to get tougher".
"There are obviously a range of tougher measures that we would have to consider ... I'm not going to speculate now about what they would be," he said.
The opposition Labour Party leader Keir Starmer said Johnson must implement a national lockdown within the next 24 hours.
"What we've learnt is that the longer you delay the difficult decisions, the worse it is on the health front, the worse it is on the economic front," Starmer told reporters.
Scottish leader Nicola Sturgeon said her cabinet would meet on Monday to discuss possible further steps to limit the spread of the virus, and ordered Scotland's parliament to be recalled.
Britain recorded 54,990 new cases of the virus on Sunday and has registered more than 75,000 deaths from the pandemic.
The government's response has been heavily criticised. But the rollout of vaccines is set to accelerate on Monday with the first 530,000 doses of the newly approved Oxford/AstraZeneca vaccines ready to be administered, Johnson said.
He hoped "tens of millions" would be treated over the next three months.
Millions of pupils are due to return from their Christmas holidays on Monday, and Johnson advised parents to send their children to school in areas where rules allow it.
"There is no doubt in my mind that schools are safe, and that education is a priority," he said.
Some local authorities and unions have warned against reopening schools and have threatened to act against government advice. Others say closures also have a big negative impact on students.
"We must renew and maintain the consensus that children’s time out of school should be kept to the absolute minimum," Amanda Spielman, chief schools inspector, wrote in the Sunday Telegraph.
