Government scientists warned ministers of a "significant risk" in allowing Covid cases to rise by scrapping restrictions all at once.
Boris Johnson tonight announced almost all Coronavirus laws would be scrapped from step 4 of the roadmap - which is expected to take effect on July 19.
The PM and Chief Medical Officer Chris Whitty admitted this would likely lead to an increase in cases and deaths.
And Mr Johnson has repeatedly said his decision were underpinned by scientific advice, and driven by "data, not dates."
But in a document published today, SAGE scientists warned the impact could be more pronounced - even if the vaccination programme reduced the number of deaths
"There is significant risk in allowing prevalence to rise, even if hospitalisations & deaths are kept low by vaccination," the document reads.
"If it were necessary to reduce prevalence to low levels again...then restrictive measures would be required for much longer."
They suggested maintaining measures intended to minimise the spread of the virus - such as masks and social distancing - would make it easier to spot outbreaks "in advance of them growing large".
Scientists said lower infection rates would reduce the impact of "long Covid" on the NHS, and allow more capacity to be used for routine care.
They warned the poor and people from a minority ethnic background have higher risk of infection and lower vaccination rates - and that this meant an increase in infections is likely to "increase health inequalities in Covid-related illness and death."
Outlining the case for maintaining some measures, they wrote: "Even beyond the point when all adults have been offered the vaccine, keeping some level of measures in place both through summer and beyond would significantly decrease ongoing transmission.
"It is notable that countries (e.g. New Zealand) that have near-zero Covid-19 have decided to retain some baseline measures (e.g. wearing of masks on public transport) to reduce the impact of occasional outbreaks."
Modelling shows it is "highly likely" there will be another spike in infections after step 4 is introduced, because not everyone will be vaccinated, vaccinations are not 100% effective, and the virus will continue to circulate.
They also warned lifting restrictions "may recreate the conditions for super spreader events".
A chart mapping the possible interventions that could be maintained after step 4, the use of face coverings and physical distancing were both rated as "medium".
Announcing the changes at a Downing Street Press Conference, the Prime Minister said: “If we can’t reopen our society in the next few weeks, when we’ll be helped by the arrival of summer and the school holidays, we must ask ourselves - when will we return to normal?”
He said we must "balance the risk" of the disease from the virus and the harm from continuing with legal restrictions which "inevitably take their toll on people's lives and livelihoods, on people's health and mental health".
He added: "And we must be honest with ourselves that if we can't reopen our society in the next few weeks, when we will be helped by the arrival of summer, and by the school holidays, then we must ask ourselves 'when will we be able to return to normal?'
"And to those who say we should delay again - the alternative to that is to open up in winter when the virus will have an advantage, or not at all this year."
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Labour leader Keir Starmer said relaxing almost all restrictions at the same time was “reckless.”
“The announcement today is all about headlines, it’s not about public health,” he said.
“The Prime Minister said he would be driven by data, not by the dates. He doesn’t have the data yet, he won’t have the data until later this week, so he’s not in a position to take a decision until next Monday - so you have to wonder what today’s announcement is all about.
“It’s about party management, rather than the public interest.”
He added: “To throw off all protections at the same time when infections are going up is reckless.
“We need a balanced approach. We need to keep key protections in place, including masks, including ventilation - and crucially, something we’ve been asking for throughout the pandemic, proper payments to those who need to self-isolate.”