Lockdown curbs in England will remain in place for an extra four weeks in a bid to avoid a huge third wave of Covid that would cost “thousands” of deaths, Boris Johnson has announced.

The prime minister had planned to lift all remaining restrictions next Monday, June 21, but has now set a new date of July 19.

There will be a review on June 28, leaving open the option of ending restrictions on July 5 if the data proves drastically better than expected, but Johnson warned this is “unlikely”.

The four week delay is designed to give millions more people time to be vaccinated against the highly infections Delta variant of the virus.

All over-40s will have their waits between two jabs cut from 12 weeks to eight weeks, and all over-18s will be offered a first dose of the vaccine by July 19. Two thirds of the population will have had both doses by then.

Johnson acted after the latest modelling by government scientists projected that the NHS would be put under huge new pressure if England abolished all its curbs as intended on midsummer’s day.

The modelling suggested that the number of hospitalisations would match that seen in the first Covid wave in April 2020, when 3,500 people a day were admitted as patients across the UK.

The current daily hospital rate is 187, meaning the Delta variant first identified in India would spark a nearly 20-fold increase if all restrictions were scrapped later this month.

Under Johnson’s so-called “roadmap” out of lockdown, the fourth step involves the lifting of all remaining curbs including the rule-of-six guidance on gatherings, social distancing in pubs and theatres and the reopening of nightclubs.

But soaring cases of the Delta variant mean that two of the government’s four tests for unlocking – a low risk of the NHS being overwhelmed and the absence of new variants – cannot be guaranteed to be met.

The Delta variant is between 40% and 80% more transmittable than the Kent variant which triggered the UK’s second wave.

Cases of the variant are growing 70% week on week and in a third of the country numbers are doubling every week.

The average number of people admitted to hospitals increasing nationally by 15% week on week, and by 61% in the north west of England, the PM said.

In a Downing Street press conference, Johnson said there were still millions of young people unvaccinated and some elderly people were at risk even if they had two jabs.

He said that the link between infection and hospitalisation had been “weakened” but not “severed”.

“We can give the NHS a few more crucial weeks to get those remaining jabs into the arms of those who need them,” he said. “I think it is sensible to wait just a little longer.”

Johnson could face a backlash from the hospitality industry as he announced no extra financial help for those firms worried about going bust from an extra month’s delay.

In an attempt to sweeten the pill, the 30-people limit of wedding guests will be lifted.

“To give the NHS that extra time, we will hold off Step Four openings until July 19, except for weddings that can still go ahead with more than 30 guests provided social distancing remains in place, and the same will apply to wakes,” he said.

Johnson also sent a signal to his Tory backbenchers, some of whom may vote against the delay when it comes before parliament, that the extended timeline would go no further.

He pointed out that the June 21 unlocking date was always a “no earlier than” date, but the new timeframe was much stronger. “I am confident that July 19 will be a terminal date, not a ‘not before’ date,” he said.

Chief medical officer Chris Whitty said that “the assessment of risk has fundamentally shifted” since the roadmap was first announced.

Chief scientist Sir Patrick Vallance said that the entire adult population would be have had two doses of the vaccine between eight and 12 weeks after July 19, suggesting September 13th is the earliest date for nationwide full protection. The latest date would be October 11.

Johnson said he was “confident” there would be no further delay beyond the four weeks, claiming there would be “a wall of immunity” by July 19.

“Looking at the data, looking at the effectiveness of the vaccines against all variants, I think that we will be able to deliver Step Four on July the 19th.

“Now, at that stage, people may want to keep maintaining social distancing, they may want to keep being sensible. But as far as I can see, we will be in a much better position, as a country to go forward.”