The UK will step up its daily vaccination rate to ensure more than 10 million receive their second dose to provide protection against the Indian variant of Covid-19.
The move came as UK Prime Minister Boris Johnson announced a four-week delay to the final step of easing restrictions in England, which had been scheduled for June 21.
He said the choice was to press on with lockdown easing, or pause to give the National Health Service a chance to give more people vaccinations.
"By acting now we have the chance to save thousands of lives," he said.
He said July 19 was now the earliest date that lockdown conditions could be eased further.
Younger adults would also become a priority for the vaccine programme.
He is understood to view the delay as the “final stretch” as he seeks to encourage more vaccinations before rules are eased.
Health Minister Edward Argar said earlier a delay of “four weeks or so” should be enough time for up to 10 million people to receive their second dose of a Covid-19 vaccine.
He suggested there would be an increase in the daily rate of second dose vaccinations - from about 250,000 to 300,000 injections per day.
“I think 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,” he told Sky News.
“Vaccination is the key to that. I think once we’ve got those second doses in people’s arms, once we’ve got that level of protection up to around 81 per cent, I think people will be more comfortable with it and we can see more ease.”
Mr Johnson lifted a 30-person limit on weddings and will allow more outdoor seated sporting and cultural events to take place with large crowds.
However, extending restrictions for up to another month means mask-wearing, the six-person limit on indoor groups and the one-metre social distancing rule will continue.
The delay is a blow to the entertainment and hospitality sector. Many businesses have been closed or are operating under tight controls.
Senior ministers including Chancellor Rishi Sunak, Cabinet Office minister Michael Gove and Health Secretary Matt Hancock are understood to have signed off on the delay after a briefing from chief medical officer Chris Whitty and scientific adviser Sir Patrick Vallance.
The UK recorded another 7,742 new Covid-19 cases and a further three deaths on Monday.
Mr Johnson on Saturday said he felt less optimistic than two weeks earlier.
“It’s clear that the Indian variant is more transmissible and it’s also true that the cases are going up, and that the levels of hospitalisation are going up,” he said.
“We don’t know exactly to what extent that is going to feed through into extra mortality but clearly it’s a matter of serious, serious concern.”
In the past week, Britain has delivered about 170,000 first Covid-19 vaccine doses a day.
At that rate, 4.5 million more first doses could be delivered during a four-week period.
The Scottish government at the weekend announced it would shorten the interval between the first and second vaccine doses for people aged over 40 to provide more protection against the variant.
Prof Devi Sridhar from the University of Edinburgh said two doses provided as much protection as possible.
"Right now the layers of protection we have for individuals are the best tool we have to avoid serious illness and have people come into hospital," she told the BBC's Radio 4 Today programme.
“We are already seeing a third wave of cases, largely in younger people. The worry is that it will slowly move, like it has in previous waves, to older groups.”
Prof Anthony Harnden from the Joint Committee on Vaccination and Immunisation said one dose of a Covid-19 vaccine offered just 33 per cent protection against the Delta variant.
He said it “made sense” to shorten the interval between doses and that government advisers would consider extending the strategy across England.
Mr Edgar said the government was "open" to shortening the interval between doses, as it had done previously in virus hotspot areas.
"We’re really keen to make sure that everyone gets that second dose as soon as they are eligible, hence the 12 to eight weeks, partly because it’s better for everyone, but partly because we’ve seen the latest Public Health England evidence," he said.
"There will be more research done on this. With one dose you are roughly 33 per cent protected against the Delta variant - with two doses, that goes up to 81 per cent. And that’s the key to being able to safely ease restrictions."