Easing restrictions in England next week is the right thing to do, the government has insisted, as doctors said the move was a "real worry" with many still awaiting vaccination.
Health minister Edward Argar urged people to behave responsibly as lockdown measures are relaxed across England on Monday.
It comes amid mounting concern over the spread of the COVID-19 variant first detected in India, with some scientists calling for a delay in the easing and others urging the public to exercise the "utmost caution".
The variant is believed to be more transmissible than other ones – thought there is no evidence to suggest it causes more severe illness, and hospital admissions currently remain low in affected areas.
There is also no evidence that it is resistant to vaccine, though the deputy chair of the Joint Committee on Vaccination and Immunisation (JCVI), Professor Anthony Harnden, said jabs are "almost certainly less effective" at reducing transmission of the Indian variant.
The Scientific Advisory Group for Emergencies (SAGE) said the relaxation could "lead to a substantial resurgence of hospitalisations (similar to, or larger than, previous peaks)".
Minutes of a meeting on Thursday suggest there is a "realistic possibility" that the Indian variant is 50% more transmissible than the coronavirus one that emerged in Kent late last year.
The scientists said there "may be some reduction in protection" when it comes to vaccines.
And Prof Harnden said: "The vaccines may be less effective against mild disease but we don’t think they’re less effective against severe disease."
Speaking to BBC Radio 4’s Today programme, he added: "But in combination with being less effective against mild disease, they’re almost certainly less effective against transmission."
Health minister Edward Argar told Sky News: "We are confident that easing restrictions on Monday is the right thing to do, is the safe thing to do.
"We all have our part to play in behaving responsibly, as that happens, following the rules on things like mask-wearing."
The British Medical Association (BMA) said the lockdown easing is a "real worry" while many are still awaiting vaccination.
The group’s public health medicine committee co-chairman Dr Richard Jarvis said: "With key segments of the population still not vaccinated and clusters of variants, including the rapidly increasing Indian variant, becoming a growing concern, we must approach this next stage of easing lockdown with the utmost caution."
"It is a real worry that when further measures lift on May 17, the majority of younger people, who are often highly socially mobile and could therefore be most at risk of a more infectious strain, are not yet vaccinated."
At a Downing Street briefing yesterday, Prime Minister Boris Johnson said the new variant could delay the fourth stage of lockdown easing on 21 June.
The government has set out four tests that must be met for lockdowns to be eased – but Professor Christina Pagel, a member of the Independent SAGE group of experts, has said that one of these tests has not been met.
While the vaccine rollout has been successful and has helped reduce hospital admissions and deaths, and current infection rates do not risk putting unsustainable pressure on the NHS, she warned this new variant of concern increases the risks associated with relaxing the rules.
On Tuesday, she said moving to stage three of the roadmap "risks adding fuel to the fire", adding that attention should turn to ensuring the Indian variant isn’t a threat – or stamping it out.
"Personally, I would like to slow down now to reduce chance of reversing the map later risking more uncertainty, more damaging closures and longer recovery from a worse situation. We need to learn from previous experience," Prof Pagel wrote in a Twitter thread.
During Friday’s Downing Street briefing, England’s chief medical officer Professor Chris Whitty said the Indian variant is expected to become the most dominant in the UK over time.
The Scientific Pandemic Influenza Group on Modelling (SPI-M-O) has also warned it could cause a "significant resurgence" of COVID-19 because "there are still too few adults vaccinated" to stop its progress.
Four people died in the UK with the Indian variant of COVID-19 – the first known domestic deaths – between 5 May and 12 May.
Public Health England also said on Thursday there had been 1,313 cases in England of the Indian variant in a week, more than double the previous week’s figure.
Mr Johnson has unveiled plans to accelerate vaccinations among the over-50s and those who are clinically vulnerable. Instead of receiving their second dose 12 weeks after their first, the final jab will now be delivered within eight weeks.
Scotland is preparing to drop to Level 2 restrictions on Monday, meaning cinemas, theatres, concert halls and stadiums can reopen.
However, First Minister Nicola Sturgeon has announced that Glasgow and Moray will remain in Level 3 of the five-tier system for a week after both areas experienced a surge in cases.
Meanwhile, the Department of Health and Social Care has said that care home residents will be able to nominate five visitors for regular visits when lockdown eases on Monday.
Residents will be able to see a maximum of two visitors a day, who can either attend together or separately. Young children will not be included in this figure.