Boris Johnson called his new July 19 reopening date " a terminus" on Monday – but did not rule out further delays as he pushed back the full end of lockdown by four weeks.
The Prime Minister said pausing the final step of reopening in England, originally due to happen on June 21, would avoid thousands of deaths from Covid, with cases surging.
Attempting to provide reassurance of no further slippage in the schedule, Mr Johnson repeatedly said at a Downing Street press conference that he was confident full reopening would happen on July 19.
But he acknowledged that the emergence of a new Covid variant could throw the plan off course as his top scientific advisers stressed the danger would not fully disappear after the delay.
The announcement means social distancing rules, six-person limits to indoor groups, work from home guidance, face mask wearing and nightclub bans will all remain in place for now.
There are exemptions. Big weddings can return, with the 30-person attendee rule scrapped and no maximum figure replacing it, although venues must be Covid-secure and dancing is still barred.
Care home residents will also be allowed to spend a night away and not have to self-isolate for 14 days once they return.
But Mr Johnson faced an immediate backlash from industry figures, who were angered by the absence of any new Treasury financial support, and lockdown-sceptic Tory MPs.
Announcing the new date, the Prime Minister said: "As things stand, and on the evidence that I can see right now, I’m confident that we will not need more than four weeks and we won’t need to go beyond July 19."
He also described that date as a "terminus date, not a ‘not before’ date". However, he declined to give a "cast iron guarantee" that July 19 would not slip, saying he had to be "honest" with people that new Covid threats could emerge.
Mr Johnson said: "That, of course, does not exclude the possibility I’m afraid – and we have got to be honest about this – the possibility that there is some new variant that is far more dangerous, that kills people in a way that we currently cannot foresee or understand."
He said he "bitterly" regretted the delay and the impact of the pandemic had "broken everybody’s heart", but stressed he was "yearning" to lift social distancing rules.
The decision to pause reopening came after alarm in Whitehall about the Indian or delta variant of Covid, which has become the dominant strain of new infections in the UK.
The latest scientific analysis suggests the variant is between 40 per cent and 80 per cent more transmissible than other Covid strains.
Where is the Indian (Delta) variant in the UK?
The decision was taken as government scientific modelling suggested the emerging third wave of Covid cases could see as many people end up in hospital daily as the first wave.
Downing Street argued that the four-week delay would allow millions more people to be vaccinated, with the rollout strategy tweaked in an attempt to maximise protection.
The target for offering the first vaccine dose has been brought forward from the end of July to July 19, meaning all adults should have been offered at least one jab by the new final reopening date.
People in their forties will also now only have to wait eight weeks rather than 12 weeks between the first and second jab, which provides maximum immunity. That matches the rules for people aged 50 and over.
The NHS will contact people in their 40s who are now eligible for their second jab as a result of the change.
Sir Patrick Vallance, the UK’s chief scientific adviser, noted it may not be until September or October that everyone over 18 is offered their second dose of the vaccine on current plans.
How many more could we vaccinated with the delay to June 21?
A check two weeks into the four-week delay would be conducted to see whether the Covid situation had improved, but Downing Street officials said it was "unlikely" that the plan would change then.
The legislation that enshrines in law the reopening roadmap, which was first published in February, will be extended to July 19 to cover the delay. A vote in Parliament is expected this week.
Tory MPs sceptical about lockdowns have already threatened to vote against the extension, but it is expected to easily pass in the Commons given Labour’s past support for such measures.
Mark Harper, the Tory MP who leads the lockdown-sceptic Covid Recovery Group, said: "There’s nothing new in the data that we didn’t know when the Prime Minister was happy to proceed with June 21.
"It’s very concerning that the Prime Minister couldn’t rule out either a further delay beyond July 19 or the imposition of further restrictions in the autumn and winter."
No changes to the travel rules were announced. A review of the Covid border rules is due later this month, with checks on whether new countries can be added to the green list – for which no quarantine on return to the UK is required – happening every three weeks.
The Treasury is not announcing any new financial support despite the four-week delay, to the frustration of business leaders who have demanded extra help.
From next month, the Government will cover only 70 per cent of wages of furloughed workers instead of 80 per cent, with businesses having to pick up the extra 10 per cent.
Hospitality and leisure companies will also have to start paying a third of their business rates bills from the start of July, ending more than a year of their rates being waived.
A Treasury source argued that more than £1 billion of grants were still available from local authoritiesfor companies – especially nightclubs which cannot open – affected by the pandemic. "We need to keep a balance in order to ensure we can recover strongly," said the source.