Boris Johnson will press ahead with lifting most remaining Covid restrictions on 19 July, despite warnings from the government’s scientific advisers that the “exit wave” could result in more than 200 deaths a day and thousands of hospitalisations.

Speaking at a Downing Street press conference, the prime minister urged “extreme caution” on Monday, as he gave the final confirmation that step four of the roadmap would go ahead next week.

It will mean nightclubs can reopen, social distancing rules will be abandoned and mask-wearing will no longer be legally enforceable, in a “big bang” approach the shadow health secretary, Jonathan Ashworth, called “pushing down on the accelerator while throwing off the seatbelt”.

But in a marked shift of emphasis from a week ago, when he said mask-wearing would become a matter of “personal choice”, Johnson said he would “expect and recommend” that face coverings be worn in crowded indoor spaces, and people were advised to limit their contact with those outside their households.

He warned businesses such as nightclubs and theatres that it was their “social responsibility” to check their customers’ Covid status; and said he expected the return to offices to take place only “gradually”.

Government documents said that ministers will “consider mandating” the NHS Covid Pass if “sufficient measures are not taken to limit infection”.

The health secretary, Sajid Javid, had earlier told MPs: “This is not the end of the road. It’s the start of a new phase of continued caution, where we live with the virus and manage the risks.”

Under questioning from the health select committee chair, Jeremy Hunt, Javid declined to rule out imposing new restrictions in the future, if ministers judged it to be necessary.

Government advisers expect about 1,000 to 2,000 daily hospital admissions over the summer as restrictions are lifted, and 100-200 deaths a day under what was described a “central scenario”. This would depend on how quickly the public’s behaviour returns to normal.

Minutes published by the Scientific Advisory Group on Emergencies (Sage) also highlighted the threat of a new vaccine-resistant variant emerging in the UK, which they warned would pose a risk to the whole world.

Johnson said there would be an increase in cases whenever restrictions were lifted, and it was better to do so now, with the “natural firebreak” of the school summer holidays, than during the autumn or winter when the NHS is already under intense pressure.

“But it is absolutely vital that we proceed now with caution, and I cannot say this powerfully or emphatically enough – this pandemic is not over,” he said. “This disease, coronavirus, continues to carry risks for you and your family. We cannot simply revert instantly on Monday 19 July to life as it was before Covid.”

Asked if the government’s message in recent days had been confusing, Johnson avoided the question and again stressed the need for a slow approach: “Yes, this is a big package of measures we’re announcing, but it will only work if people are cautious in the way they go ahead with it.”

Chris Whitty, the chief medical officer for England, said people should avoid a rush back to work, keep away from crowded areas, and be wary about unnecessary meetings.

“It’s a message about going steadily, and that’s what people have been doing all the way through this,” Whitty said. “That’s all we’re really saying – carry on being steady.”

Modelling released by Sage highlighted the potential increase in pressures on the NHS amid fast-rising infection levels and the risks from long Covid, and made clear the pivotal role played by public behaviour.

The models, produced by a number of different academic teams, suggest there could be at least 1,000 hospital admissions a day at the peak of the current wave.

However, should pre-pandemic behaviour bounce back rapidly over a month, models from researchers at the University of Warwick suggest there could be 1,300 to 4,800 hospitalisations a day. In some scenarios, particularly where admissions are lower over this summer, the experts also suggest there could be another wave in the winter.

“Most modelled scenarios have peaks lower than January 2021. However, under more pessimistic assumptions, some scenarios show a resurgence of that scale or larger. Even if lower than previous peaks, the number of admissions may become challenging for the NHS,” minutes of a Sage meeting note, adding that contingency plans should be put in place for how to respond if hospital admissions approach such levels.

However, Sage adds that while hospital admissions are a key concern, they are not the only major risk, with increased cases of long Covid, workforce absences and new variants also flagged.

“The combination of high prevalence and high levels of vaccination creates the conditions in which an immune escape variant is most likely to emerge,” the Sage minutes state. “The likelihood of this happening is unknown, but such a variant would present a significant risk both in the UK and internationally.”

With infection levels high, they add, testing capacity and resources to identify the variant involved may become overwhelmed, making it harder to quickly spot a new variant.

NHS bosses warned ministers that the service’s efforts to tackle its huge backlog of care would be hit, and people seeking normal care would have to wait longer, if hospitals have to focus once again on caring for an influx of Covid patients.

Speaking alongside the prime minister, Sir Patrick Vallance, the government’s chief scientific adviser, said that while a rise in infections was both expected and inevitable whenever restrictions were moved, the scale of this, and the subsequent impact on hospitalisations and deaths, depended on the public.

He said: “If behaviour returns immediately to pre-pandemic levels, that would be a very, very big rise. If we go slowly and cautiously, that will be less of a rise.”

Official guidance issued at the same time as the announcements stressed the expectation of limited change in people’s behaviour – and held out the prospect of tougher restrictions being imposed again in the autumn.

The guidance says people should limit contact with those outside their households where possible, return to workplaces slowly and wear masks in crowded areas.

Another document said the government would, in September, review the country’s preparedness for the coming months “which will consider whether to continue or strengthen public and business guidance as we approach the winter”.