Boris Johnson’s “rule of six” means large families face being kept apart – as children are included in the new restrictions.
From Monday, groups of just six people from different households are legally allowed to gather indoors or outdoors in England.
Although there are exceptions, notably for weddings, funerals and work, the rule will still apply to families regardless of their size.
It means families of five can meet up with just one other person from another household.
This could force parents to make difficult choices over which relatives to exclude from gatherings.
The Prime Minister said he is sorry for having to make the change to rules, but a sharp rise in the number of new cases meant doing nothing wasn’t an option.
He said: “This rule of six will of course throw up difficult cases, for example two whole households will no longer be able to meet if they would together exceed the limit of six people and I’m sorry about that, and I wish that we did not have to take this step.
“But as your Prime Minister, I must do what is necessary to stop the spread of the virus and to save lives.
“And of course we will keep the rule of six under constant review and only keep it in place as long as is necessary.”
It is not yet clear how long the rule will remain in place.
It is expected that these rules will be in place for at least three months – and if there isn’t a major breakthrough it could be as long as six months.
Mr Johnson has previously said he hoped things could be back to normal by Christmas, but today he painted a more pessimistic picture.
Boris Johnson added that there will be some exemptions to the “rule of six”.
Mr Johnson said: “There will be some limited exemptions. For example, if a single household or support bubble is larger than six, then obviously they can still gather.
“Covid-secure venues like places of worships, gyms, restaurants, hospitality venues can still hold more than six in total. Within those venues, however, there must not be individual groups larger than six and groups must not mix socially or form larger groups.
“Education and work settings are unaffected. Covid-secure weddings and funerals can go ahead up to a limit of 30 people and organised sport will still be able to proceed.”
The new restrictions follow a surge in new infections, which have raised fears of a second spike over the winter.
Asked if Christmas was now now effectively cancelled, Mr Johnson said: “Whether we are going to get things back to normal at all by Christmas, I’m still hopeful, as I’ve said before, that in many ways we could be able to get some aspects of our lives back to normal by Christmas.”
He said that quick testing could hold the key to allowing gatherings to go ahead again – but said there was no guarantee that this can be done by the end of the year.
Within minutes scientists poured water on this hope.
Mr Johnson stated: “Through that Moonshot of daily testing – everybody gets a pregnancy-style test, a rapid turn-around test in the morning, 15 minutes later you know whether you are infectious of not.
“You may not know whether you are infected or not, but you know whether you are infectious, or not, and that gives you a kind of passport, a freedom to mingle with everybody else who is similarly not infectious in a way that is currently impossible.”
He added: “We are aiming for that. We are driving for that. As I have said…we cannot be 100% sure that we can deliver that in its entirety.”
But minutes after he announced the plans, Chief Medical Officer Chris Whitty said the technology was not yet available, warning he should not put a date on when it would be because “that’s not how science works”.
And Chief Scientific Advisor Patrick Vallance said it would be “completely wrong to assume this is a slam dunk that can definitely happen.”