Matt Hancock urged people not to “give into temptation” as a mutant strain of Covid-19 discovered in the UK plunged millions more under strict new measures.

The Health Secretary tried to remain optimistic as he announced new lockdown measures for vast swathes of the UK from Boxing Day, telling people “there are brighter skies ahead”.

Earlier, one of the Government’s key scientific advisers said the new restrictions were expected to “flatten the curve”.

Speaking at a Downing Street press conference on Wednesday afternoon, Mr Hancock said: “This Christmas and the start of 2021 is going to be tough. The new variant makes everything much harder because it spreads so much faster.

“But we mustn’t give up now, we know that we can control this virus, we know we can get through this together, we’re going to get through it by suppressing the virus until a vaccine can make us safe.”

He added: “I believe that everybody will do what is needed to keep themselves and others safe, especially this Christmas, and I know from the bottom of my heart that there are brighter skies ahead.”

Mr Hancock said the Tier 4 measures “do as much as is reasonably possible” to be effective against transmitting the new strain, but added: “It all depends on how people actually behave.”

He said: “Of course, it’s easy to see pictures of people running for trains and what have you, but actually that has been a small minority. So, let’s stick at it and not give into temptation.”

Earlier, professor Neil Ferguson, a member of the Government’s New and Emerging Respiratory Virus Threats advisory group (NervTag), warned the new mutant strain which has infected swathes of people across south-eastern England, London and beyond, was “everywhere now”, but said he expected new measures to help control it.

Prof Ferguson told the Commons Science and Technology Committee: “Schools are now shut, we are in a near-lockdown situation across the country, contact rates are lower over Christmas.

“I expect, though I hesitate to make any sort of predictions, we will see a flattening of the curve in the next two weeks.

“We will see at least a slowing of growth.”

He added: “Exactly what we will see is almost impossible to predict.

“If we start to see a significant decline over the next two weeks in case numbers overall and case numbers of this variant, I would not say we were in a great position… but it offers a little more optimism that maybe we can keep on top of this.”

Prof Ferguson said the “critical question” was what happens in January, “and the extent we want to make public health measures more uniform across the country if the new variant is everywhere”.

It’s everywhere now, we can’t stop it, but it’s still beneficial to not let it jump up dramatically because we are letting people move around

London and parts of southern and eastern England were rushed into the new Tier 4 lockdown regime at the weekend, effectively taking a scythe to Christmas plans and imposing measures similar to the previous national lockdowns amid the rise of the new coronavirus mutant strain.

Scotland and Northern Ireland have already announced new lockdowns from Boxing Day, while Wales’s tough restrictions will only be eased for Christmas Day before being re-imposed.

Prof Ferguson said there was merit in restricting travel across the UK, despite the virus’ apparent omnipresence.

He told the committee: “To some extent, every little helps.

“Christmas is associated with a lot of travel around the country, so restricting that does help you prevent the situation of suddenly increasing the frequency of the virus.

“It’s everywhere now, we can’t stop it, but it’s still beneficial to not let it jump up dramatically because we are letting people move around.”

Another fast-spreading mutant strain which has sent coronavirus rates soaring in South Africa has also now been discovered in the UK, Mr Hancock announced.