Boris Johnson will order police to enforce a strict coronavirus lockdown, with a ban on gatherings of more than two people and strict limits on exercise, as he told the British public: “You must stay at home.”

The prime minister ratcheted up Britain’s response with an address to the nation on Monday evening, warning that people would only be allowed outside to buy food or medication, exercise alone once a day, or to travel to work if absolutely necessary.

All non-essential shops will close with immediate effect, as will playgrounds and libraries, he said in the address from Downing Street.

Quick guide UK lockdown: what are the coronavirus restrictions? Show Hide What do the restrictions involve? People in the UK will only be allowed to leave their home for the following purposes: Shopping for basic necessities, as infrequently as possible

One form of exercise a day – for example a run, walk, or cycle – alone or with members of your household

Any medical need, to provide care or to help a vulnerable person

Travelling to and from work, but only where this is absolutely necessary and cannot be done from home Police will have the powers to enforce the rules, including through fines and dispersing gatherings. To ensure compliance with the instruction to stay at home, the government will: Close all shops selling non-essential goods, including clothing and electronic stores and other premises including libraries, playgrounds and outdoor gyms, and places of worship

Stop all gatherings of more than two people in public – excluding people you live with

Stop all social events, including weddings, baptisms and other ceremonies, but excluding funerals Parks will remain open for exercise, but gatherings will be dispersed.

After days of being accused of sending mixed messages about what the public should do, Johnson significantly escalated his language as he urged people to comply with the more stringent measures.

“You should not be meeting friends. If your friends ask you to meet, you should say no. You should not be meeting family members who do not live in your home. You should not be going shopping except for essentials like food and medicine – and you should do this as little as you can,” he said.

“If you don’t follow the rules, the police will have the powers to enforce them, including through fines and dispersing gatherings.”

The decision follows Friday’s announcement that all pubs, restaurants and gyms should shut down, then the continuation of commuters packing into trains on Monday, and of sun-seekers crowding into parks over the weekend.

After weeks of being reluctant to copy the draconian measures seen elsewhere in Europe, Johnson announced the plan after coming under pressure from his own cabinet and Tory backbenchers as well as Labour.

“In this fight, we can be in no doubt that each and every one of us is directly enlisted. Each and every one of us is now obliged to join together. To halt the spread of this disease. To protect our NHS and to save many many thousands of lives,” he said.

“And I know that as they have in the past so many times, the people of this country will rise to that challenge. And we will come through it stronger than ever.”

It is understood that Johnson himself had until now been holding out against a lockdown, with senior advisers such as Dominic Cummings having pushed for tougher measures in the last week after initially backing a more relaxed approach.

In the past few days, there had been concern about announcing a lockdown before police had the proper powers to enforce one – it will only come into law when the emergency coronavirus legislation is passed later in the week.

Facebook Twitter Pinterest People will be allowed out to exercise once a day. Photograph: Vianney Le Caer/Rex/Shutterstock

However, Johnson has been pressured into action by the scale of worry in his own party and signs that the UK’s deaths from coronavirus are following the same pattern as Italy with a two-week lag. The total confirmed death toll rose to 335 on Monday – a sixfold increase on this time last week.

“Without a huge national effort to halt the growth of this virus, there will come a moment when no health service in the world could possibly cope; because there won’t be enough ventilators, enough intensive care beds, enough doctors and nurses,” Johnson said.

“And as we have seen elsewhere, in other countries that also have fantastic health care systems, that is the moment of real danger. To put it simply, if too many people become seriously unwell at one time, the NHS will be unable to handle it - meaning more people are likely to die, not just from coronavirus but from other illnesses as well. So it’s vital to slow the spread of the disease.”

Over the weekend, the London mayor, Sadiq Khan, had appeared to take a much clearer stance on physical distancing than the prime minister, telling the capital’s residents to stay at home. “This isn’t advice, as far as I’m concerned. These are instructions and these are rules that we should all obey to stop people dying,” he said.

The shadow health secretary, Jonathan Ashworth, welcomed the measures. “Something did have to change. We’re seeing more and more people lose their lives to this virus and the social distancing measures that the government had announced have simply not been working,” he said.

“You see beaches full of people, parks full of people, bustling markets. So we were encouraging the PM to go further this morning: we called on him to take emergency action to enforce social distancing, and it looks like that’s what he’s done this evening. So we do support it, and the drastic measures he’s taken tonight.”

Some in policing are dubious whether they currently have the powers to enforce a lockdown. One source said police were being told they would be given the powers within days, possibly being included in the emergency laws being passed through parliament now.

It is seen as anathema to the British style of policing by consent, and officers being citizens in uniform: “There is absolute reluctance,” said another source. “It’s a total change of policing style.”

The chair of the National Police Chiefs’ Council, Martin Hewitt, said: “We are working with the government and other agencies to consider how these new rules can be most effectively enforced.”

The NPCC quickly squashed suggestions that police would be assisted by troops on the streets or road blocks deployed to enforce the lockdown. A spokesperson told the Guardian: “Neither are being considered in any way.”

The national chair of the Police Federation of England and Wales, John Apter, said: “The practicalities of policing this lockdown will be challenging … policing will do all it can to keep the public safe, but we need the public to support us. I ask that the public heed the advice and stay at home unless absolutely necessary.

“This will allow police officers to concentrate on keeping the streets safe and deal with all the regular calls we receive. This is about saving lives and supporting our NHS. I ask that the public gives us their support in this time of crisis.”

Many Tory MPs are worried that No 10 has not moved fast enough to enforce stricter distancing measures, amid public alarm about evidence of people ignoring advice.

Jeremy Hunt, the former health secretary, had a stark message for No 10 that a lockdown rule was needed immediately if the UK was to have any chance of avoiding a situation like that in Italy, where the health service is overwhelmed and many hundreds are dying every day.

Coronavirus: how do Italy and the UK compare? Read more

“It may be too late to avoid Italy. But to have any chance, we need to move to lockdown rules now that ban non-essential travel. It’s time to stop asking people to do social distancing – we must enforce it. Not this week, not tomorrow, but today,” Hunt said.

As the prime minister’s statement was broadcast, MPs were debating emergency legislation giving the government the powers to ban gatherings, forcibly quarantine suspected sufferers, and compel schools to remain open, for a period of two years.

Ministers announced on Monday that the legislation could be reviewed by MPs every six months – a significant concession aimed at preventing a potential rebellion.

Labour had broadly supported the legislation, but sought more regular scrutiny of its sweeping powers. MPs debating measures sat widely dispersed on the green benches of the House of Commons, with many choosing to stay away.

After the compromise amendment was announced, Labour agreed to allow the legislation to pass without being forced to a vote.