Boris Johnson has announced a four-week delay in lifting lockdown restrictions in England to 19 July amid rising cases of the Delta variant.
The PM said the postponement of step four of the government's roadmap, which was due to take place on 21 June, will allow for more people to receive their second jab before further easing of restrictions takes place.
By 19 July there will be a "very considerable wall of immunity around the whole of the population", he added.
The proposed extension will be reviewed to see if action can be taken two weeks sooner on Monday 5 July.
MPs will get to have their say on the delay in a Commons vote, the PM's spokesperson has said.
The government believes the four-week delay could save thousands of lives.
The time between vaccine doses has also been shortened for people aged over 40 - from 12 weeks to eight weeks - with all in this age group to be offered a second dose by the week of 19 July.
This will mean two-thirds of adults will have been offered both doses by that date. All over-18s will have been invited for their first jab by 19 July.
Speaking at a Downing Street press conference, the PM said it is "sensible" to put back the end of all legal limits on social contact to 19 July and that he is "confident" no further delay will be necessary.
"We can simply keep going with all of Step 4 on 21 June, even though there is a real possibility that the virus will outrun the vaccines and that thousands more deaths would ensue which could otherwise have been avoided.
"Or else we can give the NHS a few more crucial weeks to get those remaining jabs into the arms of those who need them," Mr Johnson said.
"And since today I cannot say that that we have met all our four tests for proceeding with Step 4 on 21 June, I think it is sensible to wait just a little longer.
"We will monitor the position every day and if, after two weeks, we have concluded that the risk has diminished then we reserve the possibility of proceeding to step four, and a full opening, sooner."
Data suggests that the Delta variant is between 40% and 80% more transmissible than the Alpha variant and is rapidly driving up case numbers, No 10 said.
Cases are increasing by around 64% each week. Hospitalisations are also starting to rise, they added.
Later in the Commons, health secretary Matt Hancock said that additional support would be extended to Birmingham, Blackpool, Cheshire East, Cheshire West and Chester, Liverpool City Region and Warrington as cases rise. It will mean surge testing and guidance to millions in those areas to minimise travel.
At the prime minister's briefing England's chief medical officer, Professor Chris Whitty, warned the NHS could "run into trouble" if the number of people being admitted to hospital with coronavirus continues on an "exponential path".
He added that delaying the lifting of coronavirus restrictions in England will "reduce significantly the risk of a very high peak" in infections in the coming weeks.
The decision to postpone the easing of measures comes as new analysis by Public Health England (PHE) shows for the first time that two doses of COVID-19 vaccines are highly effective against hospitalisation from the Delta variant.
The Pfizer-BioNTech vaccine is 96% effective against hospitalisation after a second dose, with the Oxford-AstraZeneca jab 92% effective.
Despite the delay in lifting restrictions, the government has removed the 30-person limit for weddings, receptions and wakes - and the number of people who can attend such events will now depend on individual venue capacity.
Venues will conduct their own risk assessment on the number of people that can safely attend.
Rules on social distancing, singing and dancing will remain in place, with guests told to sit on tables of up to six people and receptions to be table service only.
Dance floors are forbidden inside, while for private ceremonies outdoors, dancing will not be illegal, but will be strongly advised against.
For weddings taking place inside private homes, the limit of six individuals will remain, with an exception in place for deathbed ceremonies where 30 people can attend.
The PM said: "We are saying that weddings can go ahead with more than 30 people, we are lifting that restriction on 30 people from 21 June providing social distancing is observed."
The requirement for care home residents to quarantine for 14 days after a visit outside of a home has also been scrapped, however patients will still have to isolate for two weeks following a hospital visit.
Elsewhere, the government announced more pilot mass events will take place within the next four weeks, with Euro 2020 matches to be included in the trials.
Under the PM's new plan, the guidance that "everyone who can work from home must do so" remains in place until step four is enacted.
Pubs remain restricted to table service and theatres and other indoor venues will be remain capped at 50% capacity.
There will be no changes to the furlough scheme despite the four-week delay, the prime minister's spokesperson confirmed.
He added that there is "no suggestion of re-imposing or moving back down through the roadmap" but that the government is keeping the data constantly under review.
Shadow health secretary Jonathan Ashworth said Labour will support the extension of restrictions but said they are only needed as Mr Johnson "left the backdoor open and let the [Delta] variant wash up on our shores".
"Tonight's announcement was predicted and predictable and will come as a massive blow to businesses across the country, to people who've done the right thing going to get their vaccination.
"But we're in this state because BJ failed to protect our borders: our borders were about as safe as a sieve. We had a weak puny border policy that allowed this variant to get into the country and then spread," Mr Ashworth said.
Labour Mayor of London Sadiq Khan said the delay will cause many business to "worry".
"The Government must now act urgently to extend economic support to all businesses who need it - including extending the full business rates holiday, the evictions moratorium and the existing furlough scheme until all restrictions are lifted," he said.
The delay has been described as a "catastrophe" for the theatre and live entertainment sector.
In a statement, Sir Howard Panter and Dame Rosemary Squire of Trafalgar Entertainment said: "This delay is yet another bungle from a government that wouldn't be given a single star in a review of its performance. The confusion and muddled-messages are reminiscent of a West End farce."
Julian Bird, chief executive of the Society of London Theatre and UK Theatre, added: "The proposed four-week delay to full reopening of live entertainment venues will have serious implications for many theatres and performing companies around the country."
And the Association of Independent Festivals (AIF) is calling for urgent intervention from government.
AIF CEO Paul Reed said in the event of mass cancellations, UK festivals will require a swift and comprehensive rescue package and targeted contingency fund that can be accessed from July 2021 to be saved.