Johnson said Monday that the "continuing effectiveness" of the vaccine rollout allows England to consider loosening restrictions, rather than tightening them, as cases rise.
"I want to stress from the outset that this pandemic is far from over," Johnson told a news conference. "It certainly won't be over by [July] the 19th," he said.
"We're seeing cases rise fairly rapidly," Johnson added. "There could be 50,000 cases detected per day by the 19th, and again as we predicted we're seeing rising hospital admissions, and we must reconcile ourselves sadly to more deaths from Covid."
Johnson's announcement comes two weeks before the planned date for lifting all restrictions in England. The prime minister added that a final decision on unlocking would be made on July 12, after considering data.
As part of the lifting of restrictions -- dubbed "Freedom Day" -- there would be a move away from legal restrictions to personal responsibility, said Johnson.
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This meant lifting of laws on face coverings, social distancing, and instructions to work from home.
"I don't want people to get de-mob happy," said Johnson. "This is still far from over."
But he added that this was on balance the time to reopen, given the summer season and school holidays.
"If we can't open in the next few weeks ... then when can we?" Johnson said.
Meanwhile the number of infections is predicted to rise if measures are rescinded, but the government believes its
vaccination campaign
has weakened the link between cases and hospitalizations and death.
Around 86% of the UK's adult population has received a first dose of a Covid-19 vaccine and more than 63% have received a second dose, according to government figures. On Sunday, the UK recorded 24,248 new cases and 15 coronavirus-related deaths.
Current restrictions include maintaining a distance of "one-meter plus," the use of face coverings on public transport and inside enclosed public spaces, a cap on the number of attendees at weddings and funerals, the continued closure of nightclubs, and scanning in and out of cafes and restaurants.
Pedestrians wear face masks while walking along Oxford Street in central London on June 6.
'A personal choice'
As a fresh wave of infection continues to swell in the UK, concern is growing among health care experts over the impact of a relaxation of pandemic measures.
Deepti Gurdasani, an epidemiologist and senior lecturer at Queen Mary University of London, said the government's decision to press forward with England's unlocking was "unsurprising" and would put significant pressure on hospitals once more, in addition to exposing many more people to "long Covid."
"The government has consistently ignored the advice and prioritized short-term economic gain," Gurdasani told CNN. "Even at the current rates, we're in for trouble, and opening up further of course increases those risks greatly. Not only would I say we need to really pause opening up further until we vaccinate many more people, but also deal with the current wave."
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Gurdasani described the government's plan as "hugely unethical" and alarming, given "we know that long Covid is common in young and healthy people."
"This is not the flu, like Sajid Javid seems to suggest," Gurdasani said. "Please tell me when flu has led to 400,000 people having chronic disability in a period of 16 months ... why would we want to expose so much of our population to herd immunity through natural infection when we have safe and effective vaccines that could be given to them in the coming weeks."
Gurdasani urged the government to adopt smarter policies like continued mask use and investment in ventilation in schools and workplaces, while allowing more time for greater vaccination coverage before lifting measures.
Pivoting to protective measures as the country emerges from restrictive lockdowns is an approach echoed by the British Medical Association (BMA), which is urging the government not to "throw progress away" and maintain some targeted methods to limit the spread of Covid-19 after July 19.
Dr. Chaand Nagpaul, BMA council chair, said in a statement it "makes no sense to remove restrictions in their entirety in just over two weeks' time" and called on ministers not to rush to meet their self-imposed deadline.
"We have made excellent progress with both the vaccination campaign and individual action from people across the country over the last 18 months, and the government must absolutely not throw this away at this critical juncture," he said.
Nagpaul said that although hospitalization numbers were not spiking as high with previous peaks, spiraling levels of community transmission "provides a fertile ground for new, potentially vaccine-resistant variants to develop."
He also added that evidence suggests one in 10 people suffer longer-term impacts of long Covid, even if they only suffered a mild infection, and that an estimated 2 million people in England were living with long-lasting symptoms.
"These factors could have serious consequences for the NHS and public health teams as well as business, education and wider society -- therefore stemming the spread of the virus in the community with a series of manageable, targeted measures must be the priority right now," Nagpaul said.
'An abdication of responsibility from the government'
Stephen Griffin, an associate professor in the School of Medicine at the University of Leeds, said in a statement provided to the UK's Science Media Centre (SMC) that vaccines provide "a clear route" out of the pandemic.
He added, however, that the "impatience with which restrictions are due to be relaxed is likely, in my view, to greatly amplify the number of infections we see caused by the Delta variant, and so cause unnecessary harm along this road."
Griffin added that Johnson's plan, which effectively shifts the responsibility for safety measures from the government to individuals, represented "an abdication of responsibility from the government."
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But not all medical professionals are in agreement, with others saying the argument for removing all restrictions was compelling.
Paul Hunter, a professor in medicine at the University of East Anglia, said in the same SMC statement that "there is a general consensus that Covid will never go away" and lifting restrictions over the summer -- when schools are out -- was the right move.
"Even though case numbers are rising quite rapidly at present, possibly as a consequence of celebrations around the Euros [soccer tournament], I still think it would be safer to lift restrictions now than in the autumn. The disease burden associated with a larger peak during the summer would likely be less than one during the winter," he said.
"Of course we have seen new issues appear during the course of this epidemic and so no one can be certain of the challenges over coming months, but we will eventually come into an equilibrium with this virus as we have with all the other endemic respiratory infections."
CNN's Lauren Kent and Sheena McKenzie contributed to this report