Boris Johnson said it was the right time to update the Government’s message in England at least from stay at home to stay alert, which has been criticised by those who say it lacks clarity.
Mr Johnson's address to the nation on Sunday came as the leaders of Scotland, Wales and Northern Ireland rejected his new "stay alert" slogan, preferring to keep the "stay home" message in place.
But he was to set out the "first careful steps" for easing the lockdown while keeping social distancing in place to prevent a spike in Covid-19 cases that would overwhelm the NHS.
The Government is actively encouraging people who can't work from home to go back to work if they can from tomorrow, as he seeks to limit the economic damage.
But he told them not to use public transport as step one of a three step plan.
Boris Johnson said: “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.”
From Wednesday in England people will be able to take unlimited exercise or drive somewhere but only with members of the same household.
The Prime Minister said: “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.”
As part of step two there will be a phased reopening of shops and primary schools in England, initially reception , year one and year six, in June at the earliest.
He said: “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 on step three - at the earliest by July - and subject to all these conditions and further scientific advice, he added: “ 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.”