People are being urged to act now to tackle rising rates of coronavirus as testing for secondary school-age children in the worst-affected parts of England was announced.
The country’s chief medical officer said increasing rates of infection in the South East were “concerning” and encouraged people to stick to the guidance.
Professor Chris Whitty said a third wave is not inevitable but insisted people must be “very, very sensible” over the Christmas period.
Health Secretary Matt Hancock said he was “particularly concerned” about the number of cases in London, Kent and Essex, and that the data showed “by far” the fastest rise was among 11 to 18-year-olds.
He said experience has shown that a sharp rise in cases in younger people can lead to a rise among more vulnerable age groups.
He told a Downing Street briefing: “We need to do everything we can to stop the spread among school-age children in London right now – we must not wait until the review, which will take place on December 16. We need to take targeted action immediately.”
Rising cases in London have led to fears it could be moved up into the toughest coronavirus restrictions next week, something Mayor Sadiq Khan said would be “catastrophic” for hospitality in the capital.
Public Health England (PHE) data analysed by the PA news agency shows case rates are currently rising in a majority of local areas in England, including 31 of 32 London boroughs.
Mr Hancock said he had spoken to the leaders of London’s councils and the mayor, and they had “decided to put in place an immediate plan for testing all secondary school-aged children in the seven worst affected boroughs of London, in parts of Essex that border London and parts of Kent”.
He added: “We want to keep schools open because that is both right for education and for public health.”
He said they were “surging” mobile testing units and working with schools and local authorities to encourage children and their families to get tested – whether they have symptoms or not – over the coming days.
In Wales, secondary schools and colleges will move to online learning from Monday following advice from the Welsh chief medical officer that the public health situation in the country is “deteriorating”.
The testing announcement came as financial watchdogs strongly criticised the Government’s handling of the Covid-19 Test and Trace operation and insisted it must do better.
An interim study by the National Audit Office (NAO) found that not enough test results had been delivered within 24 hours, and too few contacts of infected people were being reached and told to self-isolate.
With the budget for the NHS Test and Trace Service (NHST&T) for 2020-21 soaring to £22 billion, the financial watchdogs said the situation needed to improve.
Responding to the study, chairwoman of the Commons Public Accounts Committee (PAC) Meg Hillier said: “Throwing more money at the problem clearly isn’t the answer.
“Testing capacity has risen and it’s easier to get a test locally but Test and Trace’s performance still isn’t good enough.”
In a business plan published by NHS Test and Trace on Thursday, it laid out plans to ramp up its service with serial testing aimed at cutting the isolation period for contacts of positive cases, a rollout of community testing in Tier 3 areas of England, and the possibility of regular coronavirus testing in “critical industries” and hospitality.
Despite rising cases in the South East, Prof Whitty told the No 10 briefing a third wave was “not inevitable” and people could work together to follow the rules and bring infection rates down.
Speaking about Christmas, he said: “People should really be very, very sensible over that period and over this whole period of risk because this is a very risky period for us.
“But it’s definitely not inevitable that things will get substantially worse, that’s something we all need to work together on.”
Meanwhile, more hospital sites in England are joining the Pfizer vaccine rollout, with GP-led sites due to begin vaccinations next week, and jabs to be administered in some care homes by Christmas.
The latest data from the Government showed a further 516 people had died within 28 days of testing positive for Covid-19 as of Thursday, bringing the UK total to 63,082.