Boris Johnson has said caution is “absolutely vital” before the abandonment of virtually all formal Covid restrictions as ministers toughen their language amid expectations of soaring infection rates.

The Guardian understands that ministers have been told to brace for at least one to two million new cases of coronavirus in the coming weeks, though the vaccination programme means far smaller proportions of those infected will be hospitalised and die than in previous waves.

The move into the final stage of unlocking on 19 July, to be announced by the prime minister on Monday afternoon, has been billed as the moment for people to rely on their own judgment over coronavirus precautions, rather than official prescriptions.

And while Johnson will outline the end of virtually all statutory restrictions on everything from business capacity to distancing and mask use, people will still be urged to wear masks in crowded enclosed spaces, with similar vigilance expected over the swift return to workplaces.

The move “must come with a warning”, said Johnson, who will host a Downing Street press conference at the same time as Sajid Javid, the health secretary, briefs MPs in the Commons.

“Cases will rise as we unlock, so as we confirm our plans today, our message will be clear. Caution is absolutely vital, and we must all take responsibility so we don’t undo our progress, ensuring we continue to protect our NHS.”

There has been a marked shift in the tone taken by ministers on the subject of mask-wearing amid government jitters about the risks of the so-called “big bang” approach to unlocking.

Nadhim Zahawi, the vaccines minister, stressed the need for the public to remain “cautious and careful”. He told Sky News: “The guidelines that we will set out tomorrow will demonstrate that, including guidelines that people are expected to wear masks in indoor enclosed spaces, and of course to remain vigilant.”

His comments contrast with the tone taken a week earlier by Robert Jenrick, the communities secretary, who said he would abandon mask use when the restrictions ended, adding: “I don’t particularly want to wear a mask.”

Two days later, Javid said that while he would carry a mask with him, he would not necessarily wear one even if it was recommended, for example by a train company, if the carriage he was travelling in was quiet.

The comments prompted alarm from groups representing people particularly vulnerable to Covid or who have compromised immune systems, such as blood cancer patients, with warnings that a wholesale ditching of masks by the public could mean such groups are effectively confined to their homes.

Another potential change is expected to be renewed caution over a move away from people working from home when they can, an area identified by public health experts as a potentially important factor in determining whether reopening leads to a significant rise in Covid cases.

While decisions about where people should work will be up to individual companies after 19 July, Dr Susan Hopkins, the incident director for Covid at Public Health England, said people should aim for “a cautious return to the office” even once case rates start to fall.

“If you are able to do your business effectively from home, then I think over the next four to six weeks, we should try our best to do that,” she told Times Radio.

Johnson is to stress that even if cases approach the 100,000-a-day level predicted by Javid, a record for the pandemic, the fact that almost 87% of UK adults have received at least one vaccination, and just under 66% have had two, will greatly curtail levels of serious illness and death.

Downing Street’s argument against waiting for even higher vaccination levels is that 19 July is close to the start of English school holidays, meaning that one route of transmission will be curbed for six weeks, and will increase pressure on the NHS at a time when it is not also tackling winter issues like seasonal flu.

Ministers accept that the approach represents a leap into the unknown, as younger people – many of whom have not yet received two vaccine doses – could still get longer-term Covid symptoms even if they do not fall seriously ill.

Labour has called for more mitigation measures, such as moves to help hospitality businesses improve ventilation, and mandating mask use on public transport and shops.

Kate Green, the shadow education secretary, told Sky she understood that metropolitan mayors Andy Burnham and Sadiq Khan would make masks compulsory on trams in Manchester and on the tube in London.

Another issue for ministers to consider is the strain on the NHS; a number of hospitals have already had to cancel operations because of the twin pressures of increased Covid patients and staff being obliged to self-isolate. Javid said on Sunday that the scale of the Covid-created backlog could see waiting lists rise to 13m in the coming months.

Speaking to the Sunday Telegraph, he said the situation was “going to get a lot worse before it gets better”.