The government will keep most coronavirus restrictions in place for a further four weeks after June 21, ITV News has confirmed.
Johnson will be joined by chief medical officer Chris Whitty and chief scientific officer Patrick Vallance in a briefing as he makes his announcement.
The move follows warnings from scientists that the rapid spread of the Delta variant first identified in India risks a “substantial” third wave if it is allowed to spread unchecked.
Johnson is expected to appeal to the public to show patience, with one last push to ensure that when controls do finally end it is “irreversible”.
However, it comes as a huge setback to many businesses – particularly in the struggling hospitality sector – which had pinned its hopes on a full summer reopening to recoup some of the losses of the past year.
There was deep frustration among lockdown sceptics on the Conservative benches who said there was no reason not to end the restrictions as those most at risk of death or serious illness are now fully vaccinated.
What was supposed to change from June 21?
As part of the original roadmap out of lockdown for England, June 21 was the earliest date for the fourth and final step.
Up to six people for indoor social gatherings. Up to 30 people for outdoor social gatherings.
Restaurants, cafes and pubs allowed to serve people indoors and outdoors
Most indoor entertainment venues allowed to open, e.g. cinemas, theatres, play areas and hotels
Indoor adult group sports and exercise classes allowed
Wedding and funerals limited to 30 people - includes bar mitzvahs and christenings
Music and sports events can operate with up to 1,000 people indoors or 4,000 outdoors - or half-full, whichever is the lower number
In the largest outdoor venues, like football stadiums, up to 10,000 people can attend or a quarter-full, whichever is the lower number
What is the next step?
All legal limits on social gatherings, indoors and outdoors, removed.
Remaining premises, including nightclubs, to reopen
Relaxing of limits for large events
But social distancing, hand washing and ventilation practices to continue
Former minister Mark Harper, the chairman of the Covid Recovery Group (CRG) of Tory MPs, said any postponement would be a “political choice”.
He warned that if the unlocking did not go ahead as planned, restrictions could carry on through the autumn and into the winter as other respiratory infections picked up.
“The effectiveness of our vaccines at preventing hospitalisation means unlocking on June 21 could proceed safely. Any decision to delay will be a political choice”, he said.
“Variants and mutations will appear for the rest of time. We have to learn to live with it.
“If our very effective vaccines cannot deliver us freedom from restrictions, then nothing ever will”.
Steve Baker, the CRG deputy chairman, questioned how long the country could “fumble along” with restrictions that had such “devastating consequences” for both business and people’s mental health.
Conservative backbencher Marcus Fysh said delay was a “disastrous and unacceptable policy”.
Conservative MP Peter Bone said a delay to the June 21 reopening should not happen “without really good reason” and that currently he “can’t see the evidence why we should be postponing our freedom”.
Asked if he would vote against a delay if put to a vote in Parliament, he told BBC Radio 4’s Today programme: “What I would do is listen to what the Prime Minister says, listen to the arguments, and if I’m not convinced that these restrictions are necessary then I would of course vote against it and I hope every member of the House of Commons will listen to the argument and make their minds up.
“There has to be a vote in the House of Commons. This can’t be decided by a few ministers sitting behind closed doors. It has to be an open and transparent decision.”
He said there should only be restrictions “if there is a very clear danger to society”.
Westminster City Council leader Rachael Robathan warned of the “devastating” effects on the West End if the end of lockdown is delayed.
She said: “We are all braced for a delay today in the lifting of social distancing rules – while that will be extremely disappointing and devastating for some sectors, the Government’s priority has got to be to protect people’s health.
“But central London cannot afford any slippage in the fight to revive our economy. Footfall numbers are still down, office workers are staying away in significant numbers and central areas of the city are hurting."
From July 1, the furlough scheme will be phased out further with the government's share of furlough wages to fall from 80% to 70%, and employers will be expected to contribute the 10%.
The scheme is due to finish completely at the end of September.
Retail, hospitality and leisure businesses have also been helped with a business rates holiday since last March. The Government is due to reduce the relief to 66 per cent from July 1 and it will remain in place until the end of March next year.
Health minister Edward Argar told Sky News Johnson will address the issue of support for businesses should the June 21 easing be delayed.
He also said he knows the prime minister is "very sensitive" to the issue of weddings and couples who have had to delay their special day multiple times. He said: "I know that's one of the things he will be reflecting on very carefully as he makes his decision."
He told Sky there cannot be a “zero Covid approach” and that vaccination was the key to living with the virus.
On those admitted to hospital with the virus, he said: “We are seeing some really positive news on that, although with the Delta variant we are seeing the numbers in hospital creeping up a bit, I think they were just over 1,000 at the weekend.
“But when you look back, it was something like 38,000 at the peak in January. So we are seeing that severing of the link between the disease and hospitalisations and death.
“I think that on that basis, everyone will recognise that there comes a point where we do have to live with this disease and recognise that you cannot go for a zero Covid approach, you have to live with it, and vaccination is the key to that.
“So I think once we have got those second doses in people’s arms, once we have got that level of protection up to around that 81%, then I think people will be more comfortable with it.”
After hosting the G7 summit in Cornwall over the weekend, the Prime Minister is thought to have spent Sunday evening going through the latest data with the senior ministers and officials most closely involved in the process.
The so-called “quad” of Johnson, Chancellor Rishi Sunak, Health Secretary Matt Hancock and Cabinet Office minister Michael Gove were reported to have been briefed by the chief medical officer Professor Chris Whitty and the chief scientific adviser Sir Patrick Vallance.
UK ministers, meanwhile, have insisted they remain on course to get an offer of a second dose of the vaccine – which gives significantly greater protection against the Delta variant than a single jab – to all over 50s by June 21.
However, Foreign Secretary Dominic Raab indicated at the weekend that they wanted to use the additional time to get millions more younger people double-jabbed.
He said that while the vaccines had weakened the link between infections and hospital admissions, they wanted to be sure it was “severed and broken”.
The cautious approach was, he said, necessary to ensure the the unlocking was “irreversible” and that they did not have to “yo-yo back in and out of measures”.
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The latest daily government figures from Sunday showed another rise in infections with a further 7,490 lab-confirmed cases in the UK – up 2,149 from a figure of 5,341 the previous week.
The data also had England with a total of 35,971 positive tests in the past seven days at a rate of 63.9 per 100,000 people.
Public health expert Prof Linda Bauld, from the University of Edinburgh, suggested a delay to lockdown easing may help break the link between infections and hospital admissions.
She told LBC that the Delta variant, first identified in India, now accounted for the majority of UK cases, but the death rate among people with this infection was low.
“You can see amongst people who were infected with this variant, the mortality rate was 0.7%, just 12 people,” she said.
“We think they are all the people who had underlying health conditions and died with Covid, not from Covid necessarily.
“So the proportion of people in hospital now is half of what it was, if we were in the previous situation in 2020 and 2021.”
She added: “We have weakened that link between infections and hospitalisations and death but we haven’t broken it.
“And I think we can break it or certainly have it at a much higher level, if more people have both doses."
She said a delay to lockdown easing is “proportionate” to prevent further lockdowns.
Welsh Economy Minister Vaughan Gething will give an update on the latest coronavirus developments in Wales on Monday, which the most recent UK Government figures show had new 597 positive tests in the past week at a rate of 18.9 per 100,000.
Numbers remain similarly low in Northern Ireland, which had 596 positive tests in the week to Sunday at a rate of 31.5 per 100,000.
Northern Ireland Health Minister Robin Swann warned against complacency as he said the situation in the country could change rapidly, with current modelling suggesting the potential for a surge in hospitalisations by late summer due to the Delta Covid variant.
UK figures showed Scotland had 6,035 positive tests at a rate of 110.5 per 100,000 as data released by the Scottish Government on Sunday said the country had recorded 1,036 new coronavirus cases in the past 24 hours.
Holyrood has said it will mail out some 17,000 lateral flow test kits to football fans heading to the Euro 2020 fan zone in Glasgow, where testing is not mandatory for access but has been strongly encouraged by health officials.
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