All secondary school aged children in parts of London, Essex and Kent will be  tested for coronavirus, Matt Hancock has announced.

Speaking at a Downing Street press conference on Thursday, the health secretary said “targeted action” was needed “immediately”.

Hancock said this was due to a “sharp rise” in infection rates in those areas among teenagers.

He warned the public not to “blow it” by abandoning social distancing rules just because the vaccine rollout had begun.

HuffPost UK understands MPs expect London to be moved into the toughest tier 3 coronavirus restrictions from December 19.

In a call with MPs from the capital, health minister Helen Whateley stressed that no decision had been made to move the city up from tier 2.

But MPs who saw data presented by London public health director Kevin Fenton are expecting tighter restrictions to be imposed as London now has the highest case rate in England.

London’s mayor, Sadiq Khan, has said the harshest tier 3 coronavirus restrictions would be “catastrophic” for the capital.

Hancock said the tiering decision for London was “not inevitable”, and urged people to change their behaviour in order to keep case rates down.

He said: “By far the fastest rise is among secondary school age children 11-18 years old, while the rate among adults in London is broadly flat.

“But we know from experience that a sharp rise in case in younger people can lead to a rise among more vulnerable age groups later.

“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.”

Hancock said the government wanted to keep schools open as that was “both right for education and for public health”.

The testing will be conducted in the seven worst affected boroughs of London, in parts of Essex that border the capital and parts of Kent.

More details about how secondary school children will be tested would be announced on Friday, Hancock said.

Speaking at the press conference on Thursday, chief medical officer for England Chris Whitty warned that people must be “very, very sensible” over Christmas during a highly risky period for coronavirus.

“A third wave is not inevitable,” he said. But added: “The way we prevent it is everybody, all of us, coming together and deciding we want to try and stick to the guidance that’s there.”

Whitty 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.”

The weekly rate of new Covid-19 cases in the London borough of Havering has risen above 400 cases per 100,000 people, making it one of the highest rates in England.

A total of 1,040 new cases were recorded in Havering in the seven days to December 6 – the equivalent of 400.7 cases per 100,000.

This is up from 753 new cases in the previous seven days, or 290.1 per 100,000.

Havering now has the eighth highest rate of any local authority area in England.

Along with Havering, three other London boroughs are now among the top 20 highest rates in England: Barking & Dagenham (333.5, up from 268.7); Waltham Forest (327.1, up from 214.5); and Redbridge (310.3, up from 308.3).