Boris Johnson has announced that the final stage of lockdown easing will not go ahead on June 21.
The country will instead remain at step three until Monday 19 July, in a bid to avoid a "wave of new hospitalisations," the Prime Minister said.
All restrictions, including on social distancing and mask wearing, were due to be lifted next Monday in stage four of the roadmap.
Speaking at a Downing Street press conference, Mr Johnson said the rise in new variants and the risk of a 'surge in hospitalisations' meant the June 21 date has now had to be vacated.
Despite this, an easing of the rules will come from June 21 for weddings, care homes and wakes.
The 30-person limit on both weddings and wakes will be removed in England from next Monday, meaning there will be no numerical limit either indoors or outdoors.
No changes have been made to allow large events to go ahead in a blanket way without social distancing, although more pilot schemes could be introduced by the government.
Mr Johnson announced a two-week review of the delay on June 28, with the PM's official spokesperson confirming if data is "much better than expected" restrictions could be eased on Monday, 5 July.
There is no guarantee step four will not be delayed again, and the move to step four will only be confirmed on July 12, the Prime Minister confirmed.
The PM’s official spokesman said the extension means "thousands more deaths can be prevented" as it gives time to get more people vaccinated.
The spokesman said: "Vaccine effectiveness, especially after two doses, means thousands more deaths can be prevented if more people are jabbed.
"As the Chief Medical Officer has said, at some stage we are going to have to live with this virus as we do with flu.
"But when we have effective vaccines and a variant that needs two doses for maximum protection, it is right to allow more time to save lives."
England could suffer a wave of hospital cases as bad as the first wave last April if step four went ahead on June 21, government advisers have said.
Modelling by the Scientific Pandemic Influenza Group on Modelling (SPI-M) - a SAGE subgroup - shows hospitalisations would reach around the peak of the first wave if lockdown was eased.
The government has now received advice that the Delta (Indian) variant is between 40% and 80% more transmissible than the Alpha (Kent) strain - much higher than first thought.
Nationally there are around 8,000 new cases per day, the highest since February.
Ministers have been told cases are growing at 70% week-on-week and in around a third of the country, are doubling every week.
Hospitalisations are increasing by 15% week-on-week and by 66% in the North West.
The government has officially brought forward its July 31 target to offer a first dose of vaccine to all over-18s in the England.
Instead it now aims to offer all adults a first dose of the vaccine by July 19 - the new date step four should take effect.
At the same time, ministers have set a new target to offer all eligible over-40s in England a second dose by July 19 at the latest.
To achieve this, all over-40s will now be offered their second dose eight weeks after their first - not 12 weeks, as is the case now.
The 30-person limit on weddings will be removed in England from June 21, despite step four being delayed, the Prime Minister announced.
That means there will be no numerical limit on either receptions or ceremonies - either indoors or outdoors.
Instead, capacity limits will be the highest number of people venues can accommodate while still being Covid-secure.
This is the approach that is already taken at funerals.
The 30-person limit will also be lifted on weddings outdoors on private land, including in gardens. There will be a risk assessment which organisers need to fill out.
But other restrictions on weddings - including the ban on most singing and dancing, and guests having to wear masks at certain points - will remain in place until step four.
People at an indoor venue will need to be on tables of no more than six people. Indoor venues must shut off their dance floors - apart from for the 'first dance.'
Marquees on private land need to have two sides open to qualify as 'outdoors'.
The rule of six indoors still applies to weddings in private homes - except for deathbed weddings, which can have up to 30 guests.
The 30-person limit on other commemorative events including wakes will be lifted in England from June 21.
Instead, capacity will be driven by the maximum number of people a venue can accommodate within Covid-secure rules.
This means wakes will follow the same capacity rules as funerals and weddings.
Rules on visits outside care homes will be relaxed in England from June 21, despite the delay in other areas.
The PM’s spokesman said: "The requirement for residents to isolate for 14 days after visits out of care homes will also be removed in most cases."
It is not yet confirmed which cases will still require a 14-day isolation.
However, officials indicated care home residents won’t need to isolate at all after visits outside their home, apart from possibly hospital visits.
It’s not yet known if this will allow residents to have overnight stays with relatives.
Fuller guidance is due to be published by the Department of Health and Social Care.
There are no changes to indoor hospitality rules, or any changes to allow large events to go ahead in a blanket way without social distancing.
But the government is looking at holding more events research pilots which could help theatres and similar venues in the long run.
"We appreciate the pressure that some sectors are under", No10 said.
Work from home
Advice to work from home where possible will continue to apply after June 21, due to the delay to step four.
General laws on masks in shops and on public transport remain in place, and are not being lifted from June 21.
There are no plans to reintroduce masks in schools - though there is an element of local decision making based on cases in their area, such is the case in Greater Manchester.
Local authorities retain the ability to change their approach in areas where there are sig outbreaks - but "we're confident our guidelines are the right approach”, said No10.
The government's announcement today to delay step four will require a debate in the House of Commons and will take place on Wednesday (June 16).