Boris Johnson has been forced to tear up his plan to get Britain back to “normal” by Christmas and instead impose tough new Covid restrictions after a dramatic spike in the virus among the under-30s.

The prime minister appeared alongside chief medical officer Chris Whitty and chief scientist Patrick Vallance for a live No.10 TV briefing after figures showed coronavirus infections among young people are now rising rapidly and are double the national average.

Asked directly if Christmas was “cancelled” for many families that wanted to get together, Johnson said it “breaks my heart” but new curbs were needed.

From Monday, a new “rule of six” will be introduced in law, with arrests and fines for the first time for those who gather in larger groups.

A three-month review will take place in early December, but it is unlikely rules will be relaxed and they may be tightened further.

Among a raft of measures are:

• New police powers to break up house parties of more than six people, with neighbours actively encouraged to inform on any rule breaches;
• Pubs and restaurants forced by law to keep registers of all customers, with 10pm curfews in areas of high Covid prevalence;
• Forcing airlines to require passengers to fill in locator forms before they fly
• More Border Force police spot checks of people who arrive from abroad;
• A much more limited return of fans to Premier League football matches and other live stadium sport;
• An army of newly qualified and retired environmental health officers recruited to help councils police hospitality breaches.

Two whole households will no longer able to meet if their numbers are greater than six.

Only one support bubble will be allowed, so only one set of grandparents will be able to see their grandchildren.

“It breaks my heart to insist on these restrictions upon individuals, families, grandparents,” 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.”

Johnson admitted the rules to date had become too “complicated” and it was time to simplify the public health messages and to legally enforce them.

The new crackdown is in stark contrast to the tone struck by the PM in July, when he suggested there would be a “significant return to normality” by Christmas.

Crucially, new curbs are expected to be in place until next spring but with a three-month review in early December, government insiders warned.

Whitty said: “The period between now and spring is going to be difficult. You shouldn’t just see this as a very short term thing.”

The PM said: “I’m still hopeful that in many ways we could be able to get some aspects of our lives back to normal by Christmas.” But he accepted that would not be possible without a “moonshot” plan to get mass, daily testing in place.

The national Covid rate among all ages has in the past week risen from 12.5 to about 19.7 per 100,000 – just under the level at which the UK slaps quarantine curbs on other countries.

But the detailed figures for young people are driving the rise, with the rate for 17- to 18-year-olds now at 48.1, for 19- to 21-year-olds 54.5, and 20- to 29-year-olds overall 41.6.

The rise is not just down to extra testing, as the rate of people who test positive has trebled from 2.5% a couple of weeks ago to 6.1% among 17- to 18-year-olds, 5.1% among 19- to 21-year-olds and 3.5% among 20- to 29-year-olds.

Regarding the six people rule, Johnson said: “This will apply in any setting, indoors or outdoors, at home or in the pub.

“The ban will be set out in law and it will be enforced by the police – anyone breaking the rules risks being dispersed, fined and possibly arrested.

“This single measure replaces both the existing ban on gatherings of more than 30 and the current guidance on allowing two households to meet indoors. Now you only need to remember the rule of six.”

The PM revealed that he still wanted universities to go ahead with opening to students this autumn because they were crucial to “life chances” of young people.

“My message to students is simple – please, for the sake of your education and your parents’ and your grandparents’ health: wash your hands, cover your face, make space, and don’t socially gather in groups of more than six, now and when term starts.”