The alert levels will be one to five and the higher the level, the tougher social distancing measures will have to be.
Mr Johnson said: “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.
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What are new lockdown rules?
From Monday, people who cannot work from home are being actively encouraged to go to work instead of being told to only go if they must.
But they should avoid public transport if at all possible.
From Wednesday, people are being encouraged to take unlimited amounts of outdoor exercise and even play sports, but only with members of their household.
Mr Johnson said: “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.”
On exercise and schools Mr Johnson explained: “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.”