Video report by ITV News Political Correspondent Paul Brand
Speaking at a Covid press briefing - on the day the country's reworked Tier system was revealed - Boris Johnson said: "If we ease off now we risk losing control over this virus all over again, casting aside our hard-won gains and forcing us back into a new year national lockdown with all the damage that would mean".
Mr Johnson is facing questions, and a possible revolt from his own MPs, over the reworked England tier system set to come into force when the national lockdown ends.
The strengthened Tiers leave more than 23 million people across England in the toughest Tier 3 restrictions.
Watch the government's Covid-19 briefing in full
The PM apologised for the introduction of the stricter Tier system, acknowledging the "heartache and frustration" it would bring, "especially for our vital hospitality sector."
"I really wish it were otherwise [...] There is no doubt that the restrictions in all tiers are tough and I am sorry about that," Mr Johnson said.
He insisted, however, that the rules "strike a balance" in driving down rates of infection while being less restrictive than national lockdown.
On the potential route out of the strictest restrictions, the PM said: "Your tier is not your destiny, every area has a means of escape".
Mr Johnson stressed testing programmes as a means of easing strictest measures, with regular testing helping to drive down the virus.
The PM cited the mass rapid response testing pilot in Liverpool, which he described as a "success story", with the city set to move down to Tier 2 restrictions once lockdown ends.
Mr Johnson added that he was confident the situation will have improved by the spring, telling the briefing: I am convinced that by April things will be much, much better".
The PM appeared alongside Chief Medical Officer Chris Whitty and Chief Scientific Adviser Patrick Vallance at Thursday's briefing.
Vast swathes of the country are to enter Tier 3 - and not everyone's happy. Rebecca Barry has reaction from one top Tier area:
When quizzed on the easing of restrictions over Christmas, Prof Whitty issued a bleak warning not to hug or kiss grandparents over the festive period.
"Would I encourage someone to hug and kiss their elderly relatives? No I would not. If you want them to survive to be hugged again," he said.
Speaking at the press conference, Sir Patrick admitted the previous Tier system in England had failed to decrease Covid infection rates effectively.
Paul Brand has more on the political reaction to the PM's proposals:
"The message is that the tiers worked in terms of slowing but didn’t work in terms of flattening and reversing it," he said.
The Chief Scientific Adviser said England's national lockdown "looks as if it has flattened it [the infection rate] and is sending it downwards".
He admitted, however, that the latest ONS infection data - suggesting one in 85 people in the country having coronavirus - is a number that is "very high".
Earlier on Thursday, Health Secretary Matt Hancock revealed the list of areas set to face the strictest coronavirus restrictions.
The chaotic announcement has already faced criticism with the government's online postcode checker - allowing people to check what restrictions they might face - crashing shortly after it went live.
The PM is facing opposition to the proposals from his own party too.
MPs will vote on the new tier system in the House of Commons next week, with up to 70 Tories potentially voting against the measures.
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