Speaking in a public address to the nation, Mr Johnson announced that the country "will come back from this devilish illness" if people continue to follow the government guidance.
Here is Mr Johnson's speech in full:
"It is now almost two months since the people of this country began to put up with restrictions on their freedom – your freedom – of a kind that we have never seen before in peace or war.
"And you have shown the good sense to support those rules overwhelmingly.
"You have put up with all the hardships of that programme of social distancing.
"Because you understand that as things stand, and as the experience of every other country has shown, it’s the only way to defeat the coronavirus - the most vicious threat this country has faced in my lifetime.
"And though the death toll has been tragic, and the suffering immense.
"And though we grieve for all those we have lost.
"It is a fact that by adopting those measures we prevented this country from being engulfed by what could have been a catastrophe in which the reasonable worst case scenario was half a million fatalities.
"And it is thanks to your effort and sacrifice in stopping the spread of this disease that the death rate is coming down and hospital admissions are coming down.
"And thanks to you we have protected our NHS and saved many thousands of lives.
"And so I know - you know - that it would be madness now to throw away that achievement by allowing a second spike.
"We must stay alert.
"We must continue to control the virus and save lives.
"And yet we must also recognise that this campaign against the virus has come at colossal cost to our way of life.
"We can see it all around us in the shuttered shops and abandoned businesses and darkened pubs and restaurants.
"And there are millions of people who are both fearful of this terrible disease, and at the same time also fearful of what this long period of enforced inactivity will do to their livelihoods and their mental and physical wellbeing.
"To their futures and the futures of their children.
"So I want to provide tonight - for you - the shape of a plan to address both fears.
"Both to beat the virus and provide the first sketch of a road map for reopening society.
"A sense of the way ahead, and when and how and on what basis we will take the decisions to proceed.
"I will be setting out more details in Parliament tomorrow and taking questions from the public in the evening.
"I have consulted across the political spectrum, across all four nations of the UK.
"And though different parts of the country are experiencing the pandemic at different rates.
"And though it is right to be flexible in our response.
"I believe that as Prime Minister of the United Kingdom – Scotland, England, Wales, Northern Ireland, there is a strong resolve to defeat this together.
"And today a general consensus on what we could do.
"And I stress could.
"Because although we have a plan, it is a conditional plan.
"And since our priority is to protect the public and save lives, we cannot move forward unless we satisfy the five tests.
"We must protect our NHS.
"We must see sustained falls in the death rate.
"We must see sustained and considerable falls in the rate of infection.
"We must sort out our challenges in getting enough PPE to the people who need it, and yes, it is a global problem but we must fix it.
"And last, we must make sure that any measures we take do not force the reproduction rate of the disease - the R - back up over one, so that we have the kind of exponential growth we were facing a few weeks ago.
"And to chart our progress and to avoid going back to square one, we are establishing a new Covid Alert System run by a new Joint Biosecurity Centre.
"And that Covid Alert Level will be determined primarily by R and the number of coronavirus cases.
"And in turn that Covid Alert Level will tell us how tough we have to be in our social distancing measures – the lower the level the fewer the measures.
"The higher the level, the tougher and stricter we will have to be.
"There will be five alert levels.
"Level One means the disease is no longer present in the UK and Level Five is the most critical – the kind of situation we could have had if the NHS had been overwhelmed.
"Over the period of the lockdown we have been in Level Four, and it is thanks to your sacrifice we are now in a position to begin to move in steps to Level Three.
"And as we go everyone will have a role to play in keeping the R down.
"By staying alert and following the rules.
"And to keep pushing the number of infections down there are two more things we must do.
"We must reverse rapidly the awful epidemics in care homes and in the NHS, and though the numbers are coming down sharply now, there is plainly much more to be done.
"And if we are to control this virus, then we must have a world-beating system for testing potential victims, and for tracing their contacts.
"So that – all told - we are testing literally hundreds of thousands of people every day.
"We have made fast progress on testing – but there is so much more to do now, and we can.
"When this began, we hadn’t seen this disease before, and we didn’t fully understand its effects.
"With every day we are getting more and more data.
"We are shining the light of science on this invisible killer, and we will pick it up where it strikes.
"Because our new system will be able in time to detect local flare-ups – in your area – as well as giving us a national picture.
"And yet when I look at where we are tonight, we have the R below one, between 0.5 and 0.9 – but potentially only just below one.
"And though we have made progress in satisfying at least some of the conditions I have given.
"We have by no means fulfilled all of them.
"And so no, this is not the time simply to end the lockdown this week.
"Instead we are taking the first careful steps to modify our measures.
"And the first step is a change of emphasis that we hope that people will act on this week.
"We said that you should work from home if you can, and only go to work if you must.
"We now need to stress that anyone who can’t work from home, for instance those in construction or manufacturing, should be actively encouraged to go to work.
"And we want it to be safe for you to get to work. So you should avoid public transport if at all possible – because we must and will maintain social distancing, and capacity will therefore be limited.
"So work from home if you can, but you should go to work if you can’t work from home.
"And to ensure you are safe at work we have been working to establish new guidance for employers to make workplaces COVID-secure.
"And when you do go to work, if possible do so by car or even better by walking or bicycle. But just as with workplaces, public transport operators will also be following COVID-secure standards.
"And from this Wednesday, we want to encourage people to take more and even unlimited amounts of outdoor exercise.
"You can sit in the sun in your local park, you can drive to other destinations, you can even play sports but only with members of your own household.
"You must obey the rules on social distancing and to enforce those rules we will increase the fines for the small minority who break them.
"And so every day, with ever increasing data, we will be monitoring the R and the number of new infections, and the progress we are making, and if we as a nation begin to fulfil the conditions I have set out, then in the next few weeks and months we may be able to go further.
"In step two – at the earliest by June 1 – after half term – we believe we may be in a position to begin the phased reopening of shops and to get primary pupils back into schools, in stages, beginning with reception, Year 1 and Year 6.
"Our ambition is that secondary pupils facing exams next year will get at least some time with their teachers before the holidays. And we will shortly be setting out detailed guidance on how to make it work in schools and shops and on transport.
"And step three - at the earliest by July - and subject to all these conditions and further scientific advice; if and only if the numbers support it, we will hope to re-open at least some of the hospitality industry and other public places, provided they are safe and enforce social distancing.
"Throughout this period of the next two months we will be driven not by mere hope or economic necessity.
"We are going to be driven by the science, the data and public health.
"And I must stress again that all of this is conditional, it all depends on a series of big Ifs.
"It depends on all of us – the entire country – to follow the advice, to observe social distancing, and to keep that R down.
"And to prevent re-infection from abroad, I am serving notice that it will soon be the time – with transmission significantly lower – to impose quarantine on people coming into this country by air.
"And it is because of your efforts to get the R down and the number of infections down here, that this measure will now be effective.
"And of course we will be monitoring our progress locally, regionally, and nationally and if there are outbreaks, if there are problems, we will not hesitate to put on the brakes.
"We have been through the initial peak – but it is coming down the mountain that is often more dangerous.
"We have a route, and we have a plan, and everyone in government has the all-consuming pressure and challenge to save lives, restore livelihoods and gradually restore the freedoms that we need.
"But in the end this is a plan that everyone must make work.
"And when I look at what you have done already.
"The patience and common sense you have shown.
"The fortitude of the elderly whose isolation we all want to end as fast as we can.
"The incredible bravery and hard work of our NHS staff, our care workers.
"The devotion and self-sacrifice of all those in every walk of life who are helping us to beat this disease.
"Police, bus drivers, train drivers, pharmacists, supermarket workers, road hauliers, bin collectors, cleaners, security guards, postal workers, our teachers and a thousand more.
"The scientists who are working round the clock to find a vaccine.
"When I think of the millions of everyday acts of kindness and thoughtfulness that are being performed across this country.
"And that have helped to get us through this first phase.
"I know that we can use this plan to get us through the next.
"And if we can’t do it by those dates, and if the alert level won’t allow it, we will simply wait and go on until we have got it right.
"We will come back from this devilish illness.
"We will come back to health, and robust health.
"And though the UK will be changed by this experience, I believe we can be stronger and better than ever before.
"More resilient, more innovative, more economically dynamic, but also more generous and more sharing.
"But for now we must stay alert, control the virus and save lives.
"Thank you very much."