People forming "Christmas bubbles" are being told it is "vital" that they minimise contact with others from outside their household from today.
In a message on his Twitter account, which included a link to government guidance around the easing of COVID-19 rules over the festive period, Boris Johnson said: "If you are forming a Christmas Bubble, it's vital that from today, you minimise contact with people from outside your household.
"Everyone must take personal responsibility to avoid passing the virus on to loved ones this Christmas."
Mr Johnson also warned people not to treat the allowance for three households to mix as a "target you should aim for", instead insisting that it was "very much a maximum".
"Keep it short, keep it small," advised the prime minister, who also said he hoped that vaccines would help ensure that next year looks "very, very different".
Mr Johnson said he hoped to avoid another national lockdown in England, although the prospect was not ruled out by one of his ministers earlier on Friday.
Labour leader Sir Keir Starmer has called on the PM to set out how he will avoid a third lockdown.
"Whether the prime minister rules it out or not is not the central question," he said.
"I think the central question, prime minister, is what are you doing now to prevent the chances of a third lockdown?"
And Sir Keir said limiting Christmas mixing to only two households would be a "step in the right direction".
The comments came as a nursing chief warned that the relaxation of restrictions over the festive period could trigger a "tsunami" of coronavirus cases in the new year.
However, a survey showed that half of adults across the country said they were planning to form a Christmas bubble.
In data released by the Office for National Statistics, 56% said they feel it is very easy, or easy, to understand the rules.
The figures from a survey between 10 and 13 December show that 38% of people have no plans to form a bubble over the festive period.
While more than two-thirds of England will be living under Tier 3 restrictions from Saturday, the restrictions on household mixing are set to be relaxed from next Wednesday for the festive period.
With a week to go until Christmas Day, nurses are calling on the government to give "fresh and more detailed" advice to the public in an effort to prevent a surge in COVID-19 cases in the new year.
Dame Donna Kinnair, chief executive and general secretary of the Royal College of Nursing, said: "After a difficult year, it is everybody's instinct to want to be together and see loved ones - especially those who live far apart or feel isolated.
"But what is at stake is coming into sharp focus.
"Travelling and family visits associated with this time of year will undoubtedly lead to more cases, more pressure on NHS and care services, and more deaths. By turning the second and third waves into an unrelenting tsunami, we would begin 2021 in the worst possible way."
Dame Donna added that nurses would not be able to enjoy the Christmas period "knowing what awaits them in January", and urged ministers to give more information on the risks of mixing at Christmas, saying: "This virus isn't taking Christmas off and nor should we."
The mixing of households will also likely lead to tighter measures in the new year, according to a member of the government's scientific advisory panel.
SAGE's Professor John Edmunds, of the London School of Hygiene and Tropical Medicine, told Sky News: "At the moment it doesn't look like the tier system is holding the epidemic wave back, unfortunately.
"So I think we are going to have to look at these measures and perhaps tighten them up, we really will. It's a horrible thing to have to say but we are in quite a difficult position."
But he suggested that, while the relaxation of restrictions at Christmas is "probably not good for the epidemic", it is "probably good for people's wellbeing in other ways".
He said he will not mix with elderly relatives over the festive season, choosing to wait until they have been vaccinated.
The warnings come amid growing frustration over the tier system, which saw a reshuffle on Thursday.
Health Secretary Matt Hancock moved the following areas into Tier 3 - the highest level of restrictions, which means entertainment venues and hospitality must close, other than for takeaway services:
Windsor and Maidenhead
The rest of Hertfordshire not already in Tier 3 (Dacorum, East Hertfordshire, North Hertfordshire, St Albans, Stevenage and Welwyn Hatfield)
Surrey (except Waverley)
Hastings and Rother
Bristol and North Somerset will move down into Tier 2, while Herefordshire drop into Tier 1.
All changes come into force from Saturday.
The announcement that more areas will be under the strictest restrictions led to criticism from the Conservative backbenches.
Steve Baker, the deputy chair for the lockdown-sceptic group of MPs the Covid Recovery Group, called for clarity on how the decisions are made to move areas between tiers.
He said: "After a full and damaging national lockdown, millions more people and businesses across the country are heading into tougher restrictions.
"The government must urgently clarify what the criteria are for moving areas between, and especially down, the tiers."
Stevenage MP Stephen McPartland said it was "ridiculous that we are being dragged into Tier 3", while Altrincham and Sale West MP Sir Graham Brady said the news that Greater Manchester will stay in the highest tier will be "greeted with dismay".
A new national lockdown was not ruled out by the government on Friday, after leaders in Wales and Northern Ireland announced their nations would be subject to strict restrictions after Christmas
Speaking to Sky News earlier, schools minister Nick Gibb said: "Nothing is ruled out of course as we tackle this pandemic."
But he insisted that the current tier system in England is "very effective" at finding where local spikes are and focusing restrictions on those areas.
The President of the Royal College of Emergency Medicine later said we must do "whatever it takes" to get coronavirus cases under control.
When asked whether another lockdown was needed, Dr Katherine Henderson said: "I don't really care what the terminology is, all I know is that we need to do something to get ourselves suppressing the community transmission of the virus."